The Invention and Development 

of the Radiosonde 

with 

A Catalog of Upper-Atmospheric Telemetering Probes 

in the National Museum of American History, 

Smithsonian Institution 

JOHN L. DUBOIS, ROBERT P. MULTHAUF, and CHARLES A. ZIEGLER 

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Lawrence M. Small 

Secretary 

Smithsonian Institution 
SMITHSONIA N STUDIE S IN HISTOR Y AN D TECHNOLOG Y • NUMBE R 5 3 

The Invention and Development 

of the Radiosonde, with a Catalog 

of Upper-Atmospheric Telemetering Probes 

in the National Museum of American History, 

Smithsonian Institution 

John L. DuBois, Robert P. Multhauf, 

and Charles A. Ziegler 

Smithsonian Institution Press 

Washington, D.C. 

2002 
ABSTRAC T 

DuBois, John L., Robert P. Multhauf, and Charles A. Ziegler. The Invention and Development of the Radiosonde, with a Catalog of Upper-Atmospheric Telemetering Probes in the 

National Museum of American History, Smithsonian Institution. Smithsonian Studies in History and Technology, number 53, 78 pages, 60 figures, 2002.—From a historical perspective, 

the radiosonde is one of the more significant technological innovations of the twentieth century, not only because its widespread use greatly enhanced the accuracy of weather forecasting, 

but also because some features of its basic design became the foundation of all modern analog 

telemetry systems. This study examines the way in which advances in the technology of nontelemetering balloonsondes and radio in the nineteenth and twentieth centuries culminated in 

the invention of the radiosonde in 1929. The subsequent development of radiosondes in Europe 

and the United States from 1929 to 1940 is traced in detail, when the basic design of this instrument achieved its modern form. An overview of significant modifications in radiosonde design 

after 1940 also is provided because the instruments have remained an essential meteorological 

tool in the twenty-first century. This monograph also includes a catalog of radiosondes in the 

Smithsonian Institution's National Museum of American History. Photographs of instruments 

in this unique collection that graphically depict the development stages of the radiosonde are 

presented. 

OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the 

Institution's annual report, Annals of the Smithsonian Institution. COVER DESIGN: The first true radiosonde, which was flown by Robert Bureau in 1929. 

Library of Congress Cataloging-in-Publication Data 

DuBois, John L. 

The invention and development of the radiosonde : with a catalog of upper-atmospheric telemetering probes in 

the National Museum of American History, Smithsonian Institution / John L. DuBois, Robert P. Multhauf, and 

Charles A. Ziegler. 

p. cm.—(Smithsonian studies in history and technology ; no. 53) 

Includes bibliographical references. 

1. Radiosondes—History—19th Century. 2. Radiosondes—History—20th century. 3. Radiosondes— 

Catalogs. 4. National Museum of American History (U.S.)—Catalogs. I. Multhauf, Robert 

P. II. Ziegler, Charles A. (Charles Albert), 1927- III. Title. IV. Series. 

QC879.25.D83 2002 551.63'52—dc21 2002030658 

© The paper used in this publication meets the minimum requirements of the American 

National Standard for Permanence of Paper for Printed Library Materials Z39.48—1984. 
Contents 

Page 

Preface iv 

Introduction 1 

I. THE PROBLEM OF HIGH-ALTITUDE METEOROLOGY AND THE TECHNOLOGY 

THAT LED TO THE RADIOSONDE 

1. Nontelemetering Balloonsondes, 1892-1929 3 

2. Preradio Telemetering, 1842-1894 15 

3. Radio Telemetry, 1895-1929 19 

II. THE RADIOSONDE: DEVELOPMENT AND APPLICATION 

4. The Early Years, 1921-1928 26 

5. The Prototypes, 1929-1930 30 

6. Maturity, 1931-1940 41 

7. The Radiosonde after 1940 59 

8. Conclusion 67 

III. CATALOG OF UPPER-ATMOSPHERIC TELEMETERING PROBES AND RELATED 

ARTIFACTS IN THE NATIONAL MUSEUM OF AMERICAN HISTORY, 

SMITHSONIAN INSTITUTION 

Notes 73 

Appendix: Selected United States Patents Related to Radiosondes 75 

Literature Cited 76 

in 
Preface 

"Radiosonde" is a balloon-borne instrument that transmits atmospheric data, usually 

temperature, pressure, and humidity, to a receiver-recorder on the ground. The contributions of this relatively simple device to the late twentieth-century way of life can hardly be 

exaggerated. No other factor contributed more to the systematization of weather observations, which is beneficial to all who depend upon meteorological prediction. The utilization of the radiosonde directly affected agriculture and aeronautics, and its more sophisticated offspring made possible many of the marvels of the space age. 

The collection of instruments on which this study is based was assembled throughout a 

decade and a half, 1955-1970, as part of the program of the Smithsonian Institution to establish a new museum of the history of science and technology and of the political and social history of the United States. 

A museum rarely exhibits its entire collection in any field at one time, and objects not 

exhibited are retained for reference purposes. In the latter function, nothing is more useful 

than a printed catalog. Because of the variety of museum collections, the production of 

such catalogs is only occasionally realized. Such a document is most important when the 

collection in question is uncommon, or even unique. This may be the case with the collection described herein. 

Because "old" scientific instruments often are unfamiliar even to those who use modem 

instruments serving the same, or similar, purposes, some guidance in their construction 

and functions is useful. It also is important to understand that the history of a scientific instrument is a case history in the process of invention. For these reasons, this publication 

includes a capsule history of the development of the radiosonde based upon the instruments themselves and upon the original literature connected with their development. 

We are particularly indebted to Robert Wright of the Instrument Division of the United 

States Weather Bureau, who thought old instruments worth collecting, and to Christon 

Harmantas, who knew what they were. George A. Norton, Jr., assisted greatly with the catalog of specimens. 

We also gratefully acknowledge the help of William Blair, Jr., who kindly supplied us 

with copies of selected published and unpublished documents from the collected papers of 

his father, Colonel William Blair. Thanks also are due H.C. McBrair who alerted us to the 

importance of Colonel Blair's research to the early history of the radiosonde and who 

shared with us his reminiscences of radiosonde development at Wallace and Tiernan Inc., 

an early manufacturer of radiosonde equipment. At the opposite end of the time-span with 

which we are concerned, we are indebted for the support of the following historians at the 

Smithsonian Institution: David Devorkin of the National Air and Space Museum and Deborah Warner of the National Museum of American History. 

Robert P. Multhauf 

Director Emeritus 

National Museum of American History 

Smithsonian Institution 

IV 
The Invention and Development 

of the Radiosonde, with a Catalog 

of Upper-Atmospheric Telemetering Probes 

in the National Museum of American History, 

Smithsonian Institution 

John L. DuBois, Robert P. Multhauf, 

and Charles A. Ziegler 

Introduction 

The collection and transmission of data is as old as the human species. Information about the weather may have first 

achieved "scientific" status when sensible observation (e.g., 

coldest winter in the memory of the oldest resident) of meteorological data was augmented by instruments capable of numerically describing the elements of the weather. 

The invention, in the seventeenth century, of the thermometer, barometer, hygrometer, and wind speed and direction indicators, was a phenomenon of the scientific revolution. These 

were hardly comparable in importance with some other inventions of the time, such as the telescope, whose revelations spectacularly embellished astronomy, the already crowned "queen" 

of the sciences. Astronomy already was mathematicized, 

whereas the "science" of the meteorologist still was difficult to 

distinguish from the art of the occultist. Still, instruments and 

the numerical recording of data gave hope for scientific respectability, and, by the end of the eighteenth century, the collection of meteorological data was one of the principle concerns of the numerous scientific societies that were part of the 

scientific revolution. Their periodical publications, the first of 

which was established in the 1660s, were devoted primarily to 

the dissemination of meteorological information. 

By the end of the seventeenth century, attempts had been 

John L. DuBois, C.E.O. Thermalogic Corporation, Hudson, Massachusetts, 01749. Robert P. Multhauf, Director Emeritus, National Museum ofAmericam History, Smithsonian Institution, Washington, D.C. 

20560. Charles A. Ziegler, lecturer in Anthropology, Brandeis University, Waltham Massachiusetts 02453-2700. 

made to develop meteorological instruments that automatically 

recorded their readings. Steady improvements were made in 

the data collection process, but it soon was recognized that it 

was even more important to devise some speedy method for 

transmitting the data from various collection points to a central 

bureau. One means to accomplish this task existed in the eighteenth century. The visual telegraph, which used lanterns and 

mechanized semaphores, was invented in 1794 by a French officer, Claude Chappe. Its primary purpose was to improve military communications, but because weather conditions were of 

vital military importance, both the collection and transmission 

of meteorological data increasingly were made ancillary to the 

military art. In the United States, the army played a key role in 

both the collection and the transmission of meteorological data 

via the electromagnetic telegraph, which was introduced in the 

1840s. 

Yet, by the early twentieth century, the transmission of meteorological data still was a minor application of the electromagnetic telegraph. The introduction of the wireless telegraph (radio) did little to change this, although experiments in the 

automatic transmission of data from remote meteorological instruments, including the transmission by wire from balloons, 

continued until the 1920s, when radio transmission was 

adopted. 

There were logical reasons for this lack of progress. The accumulated mass of meteorological data had not greatly improved weather prognostication, and its collection was not considered urgent. The meteorologist had not only failed, as was 

often charged, to "do something about the weather," but had a 
SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

poor record for prognostication. Meteorology remained an embarrassment to practioners of the exact sciences, who increasingly were tempted to relegate it to the closet reserved for sciences that had failed to yield to the magic of mathematization. 

It wasn't until the 1920s that there was a marked improvement in meteorology. Geophysical and weather-related problems of radio transmission had increased interest within the scientific community in the collection of atmospheric data from 

the stratosphere (Shaw, 1926, 1942; Stringer, 1972). There also 

was increased military awareness of the uses of meteorological 

data. In the civilian sector, the advantage to farmers and fishermen of improved weather prognostication and the crucial dependence upon meteorology of the new technology of aviation, 

had become apparent. Moreover, although the impetus to improve instrumental meteorology can be traced to the interests 

and needs of these segments of society, the advent of the radiosonde owed much to an almost accidental development, amateur radio—a hobby stemming from a widespread fascination 

with this new mode of communication and its "off-the-shelf 

technology. 

The efflorescence of amateur radio after World War I not 

only produced technological advances but also created a large 

market that dramatically lowered the cost of radio components. 

In the 1930s, the radiosonde was rapidly adopted by the 

weather bureaus of virtually all the industrialized nations. During the 1940s, the innovations in radio resulting from the development of the radiosonde, spurred by pressure from military 

programs, spawned the new technology of multichannel data 

telemetry. 

During and after World War II, the needs of aviation and of 

rocket engineering to capture massive amounts of data simultaneously (in "real time") from many sources were met by the 

further development of the principles and designs used to develop the radiosonde. Workers in Europe and the United States, 

who had specialized in balloonsonde or radiosonde research, 

applied their skills to develop rocket-borne instrumentation for 

atmospheric research (Kuiper, 1946:263). In the 1950s, data telemetry scientists used complex methods of radio modulation 

and transmission to enhance by several orders of magnitude the 

speed and accuracy of data transmission. Modern probes for 

exploring the upper atmosphere and space represented yet another advance in the three areas of technology originally 

merged in the radiosonde: environmental sensors, data transmission via radio, and an unmanned vehicle capable of moving 

the sensors and transmitter away from the earth's surface. 

The development of the radiosonde, from a historical perspective, is important not only because these balloon-borne instruments still are widely used today, but also because the radiosonde represents a technological link between the nontelemetering, balloon-borne probes of the pre-1930s and modern 

rocket-launched probes. Moreover, the chronological portion 

of the radiosonde's history that is the focus of this monograph 

(1929 to 1940) is especially important because it was during 

those years that the basic design features of radiosondes 

evolved, and because these developments laid the foundation 

for the introduction in the 1940s of multichannel radio transmission via subcarrier modulation (the basis for all modern analog radiotelemetry systems). 

The reason for this publication is the presence of a substantial collection of significant examples of the radiosonde in the 

National Museum of American History (NMAH), Smithsonian 

Institution. If, as appears to be the case, this is the only large 

collection of radiosonde instruments extant, it constitutes a 

unique resource for studying the history of radio telemetry and 

the history of one of the earliest applications of radio to the solution of problems in another technology. 

In order to understand the development of the radiosonde, 

the first three sections describe the relevant advances in balloon-borne instruments, preradio telemetry, and radio telemetry 

prior to 1929. The next sections outline the historical development of the radiosonde from its invention to the present. Finally, there is a catalog of the radiosondes in the collection of 

the National Museum of American History, Smithsonian Institution. 

Units are given in the metric system throughout this text. 

This has been done for simplicity and because metric units are 

standard in the field of meteorology. 

ARC 

AT&T 

NBS 

NPL 

ONR 

Plaskon 

RCA 

USWB 

WIT 

ABBREVIATIONS 

Atlantic Research Co. 

American Telegraph and Telephone 

National Bureau of Standards 

British National Physics Laboratory 

United States Office of Naval Research 

Plaskon Co., 2121 Sylvan Ave., Toledo, Ohio 

Radio Corporation of America 

United States Weather Bureau 

Washington Institute of Technology 
I. The Problem of High-Altitude Meteorology and 

the Technology that Led to the Radiosonde 

1. Nontelemetering Balloonsondes, 1892-1929 

Prior to the invention of the balloon in the late eighteenth 

century, rockets and kites were the only two vehicles capable of 

ascending into the atmosphere. The rocket was known in China 

by the early part of the thirteenth century, and it already was 

being used in Europe for military purposes before the end of 

that century. Kites, used as children's toys for many centuries 

in Asia, were introduced in Europe in the early seventeenth 

century. In 1749, British astronomer Alexander Wilson flew 

thermometers on kites to measure air temperature at high altitudes. This was perhaps the first recorded use of an unmanned 

atmospheric probe. Three years later, Benjamin Franklin performed his celebrated kite experiment on the nature of lightning. 

Rockets were quite unsuitable as instrument platforms before 

recent times; indeed this application probably was not considered. Kites also were not used often as probes until the advent 

of the Hargrave box kite in 1893. Manned flights to obtain scientific data became possible after 1783, using free balloons; 

but the captive balloon, because of its tendency to oscillate and 

rotate, was seldom used. Those defects were rectified with the 

invention of the Siegfried and Parseval kite-balloon in 1898. 

After the turn of the century, both instrumented box kites and 

kite-balloons were used by national weather services in Europe 

and the United States to obtain upper air data (Middleton, 

1969a:98). 

By 1929, a considerable amount of geophysical data had 

been gathered, using kites, unmanned captive and free balloons, manned balloons, and airplanes. The history of such research, however, lies outside the scope of this monograph and 

will be discussed only insofar as the instrumentation developed 

for use with such vehicles influenced the design of instruments 

flown on unmanned free balloons. 

The use of the unmanned free balloon to obtain scientific 

data followed closely the introduction of the balloon itself. In 

August 1783, Jacques Charles, a French Professor of physics, 

launched at Paris an improved version of the recently invented 

hot-air balloon. In this balloon hydrogen replaced heated air as 

the gas fill. The path of Charles' balloon was tracked by several observers organized by Jean-Baptiste Meusnier, a young 

army engineer, who obtained data about the atmospheric forces 

exerted on the balloon (Gillespie, 1983:32). Meusnier's work 

constituted the first practical use of unmanned and uninstrumented free balloons (later called "pilot" balloons) for scientific research. 

Throughout the nineteenth century, small pilot balloons often 

were released prior to launching a manned balloon, in order to 

estimate the winds aloft. It was not until the first decade of the 

twentieth century, however, that accurate measurements of 

wind velocity at various altitudes, using a theodolite technique 

to track pilot balloons, became an accepted meteorological tool 

(Blair and Lewis, 1931:1532). 

The practice of attaching instruments to an unmanned free 

balloon was started in France in 1892 by Gustave Hermite, 

with the help of a noted balloonist, Georges Besancon (Hermite, 1893). These early versions of what came to be called a 

"balloonsonde" used small (five cubic meter) coal-gas-filled 

balloons made of varnished paper that were capable of carrying 

self-registering thermometers and barometers. Later versions 

used balloons made of goldbeater's skin (the outer layer of the 

caecum of an ox), and, for heavier payloads containing clockwork recorders, silk balloons with volumes of several hundred 

cubic meters were used (de Fonvielle, 1899:1-22). Payload 

weight and lifting capacity were designed to ascend to about 

nine kilometers before loss of gas, through the open neck of the 

balloon and by diffusion through its skin, caused the balloon to 

descend. This technique insured a slow descent and a reasonably soft landing. Because of the prolonged floating period, 

balloons often landed as far as one hundred kilometers from the 

launch site. This dispersion exacerbated the problem of data retrieval, which depended upon the finder mailing the payload 

back to the investigator's laboratory where the recorder could 

be read and interpreted. Sometimes balloonsondes were lost, 

but even when they were found, several days or even weeks 

elapsed before the data were recovered. Despite this disadvantage, the balloonsonde was capable of returning data from 

much higher altitudes than kites, captive balloons, or manned 

free balloons (Middleton, 1969a:99; 1969b:97-102). 

The discovery of the tropopause by French scientist Teisserenc de Bort, announced in 1902, was made with a balloonsonde. The tropopause, a transition zone between the troposphere and the stratosphere, marks the point at which air 

temperature ceases to drop with increasing altitude. It begins at 

about 11.2 kilometers, an altitude considerably beyond the 

maximum heights attained by kites and captive balloons (due 

to the weight of the tether) (Teisserenc de Bort, 1899: 
SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

^ w W 

FIGURE 1.—Minimum-reading thermometer and barometer flown by Gustave Hermite in 1892. (From W. de 

Fonvielle, Les BallonsSondes, Paris, 1899.) 

417^420). The maximum height, 10.8 kilometers, that was attained by an open-gondola, manned, free-balloon in 1901, also 

was too low to discern the tropopause; it was limited by the 

maximum altitude at which humans can survive by breathing 

bottled oxygen. (The pressurized, closed-gondola balloon that 

made possible manned ascents to higher altitudes did not appear until 1931.) By 1894, some balloonsondes were returning 

data from altitudes of 18 kilometers. Thus, the balloonsonde 

constituted the only means of probing the upper reaches of the 

atmosphere during the late nineteenth and early twentieth centuries. 

The first payloads flown by Hermite involved a crude instrument (Figure 1) that provided only minimum temperature and 

pressure readings. By 1893, he was using a "barothermograph," which was capable of providing a continuous 

strip-chart record of pressure and temperature for the entire 

flight (made with special nonfreezing ink for balloon use). His 

payload was an adaptation of a laboratory device (Figure 2) 

made by the well-known instrument firm of Richard Brothers 

of Paris, in which a Bourdon tube thermometer and an aneroid 

barometer were coupled to a clockwork-driven pen-and-ink recorder. This instrument was used in a flight to almost 16 kilometers (de Fonvielle, 1899). 

The weight of the barothermograph required the use of a 

large (113 cubic meter) and costly balloon, and by 1895 Hermite was using a new device (Figure 3), which was designed 

especially for use with balloons. The pen-and-ink recorder was 

replaced by tracing points on smoked paper, and a helical bimetallic thermometer was used rather than a Bourdon tube. 

These innovations, together with generally lighter construction, 

reduced weight, cost, and complexity and increased reliability 

(de Fonvielle, 1899). 

In 1894, the year after Hermite's first balloon flight, an American meteorologist, William Eddy, obtained data using a similar 

Richard instrument attached to a kite. This flight, which was the 

first time a complex meteorograph had been lifted by kite, 

reached a height of 470 meters (Eddy, 1898:452). After this success, meteorological instruments were designed especially for 

use with kites and, later, with captive balloons. A notable example is the Richard barothermohygrograph of 1896. This design, 

originally suggested by Abbot Lawrence Rotch, founder and director of Blue Hill Observatory near Boston, Massachusetts, 

was a prototype for most of the later payload designs used for 

kites, captive balloons, and balloonsondes (Tissandier, 1896). 

The original instrument (Figure 4) was capable of measuring 

relative humidity, using a hair hygrometer, as well as pressure 

and temperature.1 This device marked the advent of the 

three-sensor system (thermometer, barometer, hygrometer) that 

remains the basis for meteorological probes today. 

Kites and captive balloons could not reach the altitudes 

achieved by balloonsondes, but they could repeatedly carry 

aloft heavy instruments without incurring much additional cost 

because the fixed expenses of the overall system, including 

motor-driven winches and cabling, were an appreciable portion 

of the cost per flight. This was not true of the balloonsonde, 

which was a complete system in itself. Hence, payload weight 
NUMBER 53 

FIGURE 2 (above).—Barothermograph made by the firm of Richard Brothers of 

Paris, 1893. The device used a Bourdon thermometer and two aneroid chambers coupled to pen-arms that traced a record on a clockwork-driven cylinder. 

(From de Fonvielle, Les Ballons-Sondes, Paris, 1899.) 

FIGURE 3 (right).—An improved barothermograph used by Gustave Hermite in 

1895. The unit used a stylus to scratch a record on a smoked paper in place of 

the pen-and-ink recordings used earlier. (From de Fonvielle, Les Ballons-Sondes, Paris, 1899.) 
SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

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it 

FIGURE 4A.—Barothermohydrograph made by the firm of Richard Brothers of Paris in 1896 for American meteorologist A.L. Rotch. (From G. Tissandier, La Nature, volume 24, 1896.) 

was reflected in the size and cost of the balloon, and this significantly affected the cost per launch. Thus, although the Richard 

barothermohygrograph was suitable for use with kites and captive balloons, lighter mechanisms were desirable for use with 

unmanned balloons. In 1896, at the request of Hugo Hergesell 

of the Strasbourg Observatory, lighter instruments (Figure 5) 

were designed by the German firm of Bosch and were used in 

an experiment in which balloonsondes were launched simultaneously in Germany, France, and Russia (Hergesell, 1897). 

This was the first international attempt to obtain synoptic meteorological data on the upper atmosphere. 

Pressure-, temperature-, and humidity-measuring payloads 

were not the only types developed and flown. As early as 1899, 

French scientist A. de la Baume-Pluvinel constructed and flew 

a sophisticated balloonsonde for solar spectroscopy (Figure 6). 

A clockwork mechanism opened and, after a brief interval, 

closed the slit of a spectrometer at a preset time during the 

flight, allowing the solar spectrum to be recorded on photographic film. During this interval, the image of the recording 

arm of a barometer was projected onto the film so that pressure 

(and altitude) could be correlated with spectral data. Perhaps 

the most innovative aspect of this balloonsonde was the use of 

a compass-like magnetic frame to align the sonde (which could 

rotate around its longitudinal axis) with the earth's magnetic 

field. The heliostat mirror was tilted slightly, so that the noontime sun's zenith distance was such as to reflect sunlight directly into the slit that was opened by the clockwork timer. This 

is the earliest example of the use of an automatic, solar, homing 

device in a geophysical probe. Indeed, conceptually BaumePluvinel's balloonsonde represented an extraordinary precursor 

of a late twentieth-century design (Baume-Pluvinel, 1903). 

Baume-Pluvinel's use of photographic film to record simultaneously the output from more than one sensor not only allowed the correlation of several parameters, but also was parsimonious in providing needed information with minimal 

equipment (advantageous in balloonsondes where low weight 

and reliability were important considerations). Moreover, it 

demonstrated that it was possible to record mechanical displacement by nonmechanical means, i.e., photographically, in a 

balloonsonde. This technique subsequently was applied to 

other types of instruments based upon the measurement of mechanical displacement, where a direct physical linkage between 

the displaced structure and a recording mechanism would be 

difficult to construct and/or would destroy the accuracy of the 

measurement (e.g., in measuring the height of mercury in a 

thermometer or the separation of the fibers of an electrometer). 

The advantageous features of the photographic technique made 

it the preferred method for investigators who sought to build 

payloads for obtaining data not available from the standard meteorological balloonsonde. 

At the turn of the century, the development of the rubber balloon dramatically altered unmanned balloon technology and 

made low cost and low weight even more important in designing balloonsondes. With the exception of the gas fill, the bal-
NUMBER 53 

FIGURE 4B.—Photo of the original version made by Richard Brothers for use with captive balloons. (Instrument 

in NMAH collection, catalog number MHT 308209; Smithsonian photo 48467-H.) 

loons used in the 1890s were essentially identical to the first 

unmanned balloon launched by Jacques Charles in 1783.2 Such 

balloons were the "fixed-volume" type, that is, the envelope 

was made of a nonstretch material that was fully inflated on the 

ground. The neck or "appendix" of the balloon was left open to 

allow gas to escape during the ascent, which prevented the expanding balloon from bursting in the rarified air at high altitude. These balloons decreased their speed of ascent with increasing altitude, and, because their descent depended upon the 

gradual loss of gas, they had a prolonged "float." Both of these 

characteristics were undesirable. The fact that the speed of ascent was not constant meant that the ventilation of sensors was 

variable, which made it difficult to interpret the data they provided. Also, the prolonged float meant that the balloons often 
SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 5.—Balloonsonde built by the firm of Bosch in 1896 for German meteorologist Hugo Hergesell. This 

represents a balloonsonde version of the Richard Brothers instrument shown in Figure 4B. (From H. Hergesell, 

Meteorologischen Zeitschrift, volume 14, 1897.) 

travelled great distances from the launch point before landing, 

making recovery costly and time consuming. 

In 1901, German meteorologist Richard Assmann revolutionized unmanned balloon technology by introducing the use 

of sealed rubber balloons (Stringer, 1972:53). These were made 

of specially processed rubber so the balloons could stretch to 

more than two and one-half times their launch diameter before 

bursting. Theoretically, this allowed them to reach altitudes exceeding 20 kilometers (a height sometimes achieved in practice). Rubber balloons provided two significant advantages 

over the fixed-volume balloons: first, because they expanded 

during ascent, they tended to exhibit a fairly constant speed of 

ascent, which made sensor data more reliable; and second, they 

promptly burst at maximum altitude. The payload quickly de-
NUMBER 53 

FIGURE 6.—Solar-homing balloonsonde built 

and flown by French scientist A. de la BaumePluvinel in 1899. The purpose of this sophisticated device was to measure part of the solar 

spectrum at high altitudes where radiation 

traverses only a small portion of the earth's 

atmosphere. (From A. de la Baume-Pluvinel, 

L'Aerophile, volume 2, 1903.) 

scended on a parachute, which considerably reduced recovery 

times. Moreover, such balloons offered two means of payload 

recovery: descent by a single parachute, which was used with 

low-cost payloads when timely recovery was important, and 

the multiple-balloon technique, which was used with costly, 

one-of-a-kind payloads when a soft landing and a high probability of retrieval were essential. The first method became standard for meteorological balloonsondes, whereas the second 

was characteristic of research flights involving hard-to-replace 

special instrumentation (for example, with the first radiosonde). 

The multiple-balloon technique (which depended upon the 

use of rubber balloons) was introduced by Hergesell in 1904. 

The single balloon and parachute combination was replaced by 

two or more smaller balloons of equivalent lifting power. Although the balloons were designed to burst at roughly the same 

altitude, it was certain that one balloon would burst before the 

others, allowing the remaining balloons and the payload to descend slowly to a soft landing. This method enhanced the likelihood of retrieval because the remaining balloon(s) floated 

above the grounded payload, acting as a signal to potential 

finders (Hergesell, 1904:200). 

Although rubber balloons were not reusable (as were balloons made of silk or goldbeater's skin), they were less costly; 

hence, the average cost per flight was not appreciably higher. 

Moreover, the new balloon technology promised even greater 

cost reduction if payloads could be lightened and rubber balloons made smaller. This in turn suggested the possibility of 

sending up large numbers of balloonsondes, making the possibility of routinely obtaining synoptic data on the upper atmosphere a meteorological reality. 

William Dines was the first to solve the payload weight 

problem (Dines, 1906:101) by eliminating the costly clockwork-driven recorder and the complicated mechanical linkages 

between sensor arm and recording arm and substituting a small, 

highly polished, flat, metal surface on which the arm of the 

sensor scratched a line (Figure 7). The total deflection was very 

small, necessitating the use of a microscope to read the recording, but this was a small price to pay for the considerable decrease in cost and weight and the increase in reliability. Similar 

designs, emphasizing simplicity and light weight, were produced in Europe and were used in balloonsonde flights before 

World War I (Middleton, 1969a: 100). 

Nonetheless, most of the upper-atmosphere data used in 

weather forecasting still were obtained from instruments flown 

on kites and captive balloons because the recorded data could 

be recovered quickly by simply reeling in the tethered vehicle, 

whereas balloonsonde records were only recovered after a considerable time. Thus, the balloonsonde was primarily a means 

of conducting geophysical research in the upper atmosphere. 

One of the more impressive examples of geophysical instrumentation was the balloonsonde (Figure 8) designed in 1913 by 
10 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 7.—Diagram of William Dines' balloon meteorograph. This device introduced the use of both a stylus 

to scratch minute displacement of sensors on polished 

metal surface and a microscope to read the resultant 

record. This approach dramatically lowered weight and 

cost, making it feasible to 

launch balloonsondes in considerable numbers. (From W. 

Dines, Symon 's Meteorological Magazine, volume 41, 

1906.) 

Charles Abbot of the Astrophysical Observatory of the Smithsonian Institution to measure solar radiation at high altitudes 

(Abbot et al., 1915:1-26). Abbot's work is noteworthy not only 

because of its scientific results but also because it demonstrated 

that the existing unmanned balloon technology was capable of 

lofting payloads to very high altitudes. 

In designing his balloonsonde, Abbot adopted the photographic technique pioneered by Baume-Pluvinel. In Abbot's 

device (Figure 8), a clockwork drive rotated a drum and also 

briefly opened the shutter of a slit-like aperture in the housing 

of the drum. Light, which passed through the aperture, impinged on photographic film on the inner surface of the drum. 

At the same time the aperture was partly covered by the mercury of a thermometer that rose or fell, depending upon the 

temperature of an attached disk pyrheliometer. Thus, the length 

of the aperture image recorded on the film measured the disk 

temperature, which was a function of the solar radiation. In order to obtain a simultaneous measurement of pressure, the image made by a barometer recording arm also was projected 

onto the film, a la Baume-Pluvinel. 

Abbot used the multiple-balloon recovery technique that had 

been introduced by Hergesell, which had become the preferred 

method for use with costly, specialized balloonsondes. Abbot intended to loft his balloonsondes to the highest possible altitudes 

so as to maximize the reliability of extrapolating the measured 

value of solar radiation to the value that existed outside the blanket of air surrounding the earth. He used special rubber balloons 

that were designed and made by a company in Russia, which 

had produced balloons capable of reaching extremely high altitudes and which had produced most of the sounding balloons 

used in Europe prior to World War I (Abbott et al, 1915). 

Abbot constructed and flew five balloonsondes of this type, 

at Avalon, California, and Fort Omaha, Nebraska, in 1913— 

1914. One flight not only obtained useful data on solar radiation, but also set an altitude record for an unmanned balloon in 

the United States (32,643 meters) (Abbott et al., 1915). This altitude was extraordinary for that era. Balloons capable of 

reaching this height generally were not available until after 

World War II. The Russian firm that made these rubber balloons did not survive World War I; indeed, even its name is unknown.3

 Thus, the secret of processing the rubber to increase 

its stretchability and its resistance to chemical attack by the relatively high concentration of ozone above 25 kilometers also 

was lost. (Present-day high-altitude balloons are made of 

ozone-resistant plastic.) 

The disappearance of the Russian firm created a serious 

problem for researchers. For example, in 1919, when Robert 

Millikan, an American physicist and future Nobel Laureate, 

constructed a balloonsonde capable of measuring environmental, high-energy radiation (later called cosmic rays) above 25 

kilometers, he had great trouble finding suitable balloons (Millikan, 1950:210). He did not obtain usable balloons from a 

United States manufacturer until 1921, but these still were incapable of reaching the desired altitude.3

 Millikan later designed a payload that was capable of measuring the intensity of 

ionizing radiation as a function of altitude to a height of 32 kilometers (Figure 9). Like the instruments of Baume-Pluvinel 

and Abbot, Millikan's balloonsondes used a photographic 

method of data storage to record simultaneously the output of a 

small, bifilar electrometer and the movement of the arm of a 

pressure sensor (Millikan and Bowen, 1926). These balloonsondes were flown in 1922, and, although the data were useful, 

the maximum altitude achieved (15.5 kilometers) was far lower 
NUMBER 53 11 

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highest altitudes achieved to that date in the United States (more than 32 km). 

(From C. Abbot et al., Smithsonian Miscellaneous Collections, volume 45, 

1915. Instrument in NMAH collection, catalog number MHT 314680.) 

FIGURE 9 (left).—Robert Millikan's balloonsonde for measuring environmental high-energy radiation (cosmic rays) flown in 1922. (From R. Millikan and I. 

Bowen, Physical Review, volume 27, 1926.) 
12 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 10.—S.P. Fergusson's balloon meteorograph designed in 1919. This represents the final development, in 

terms of low weight and cost, of the Richard Brothers design concept exemplified by the instrument shown in 

Figure 4B. In the 1920s, this approach, which still utilized a clockwork-driven recorder, was increasingly superseded by balloonsondes based upon Dines' instrument shown in Figure 7. (Instrument in NMAH collection, catalog number MHT 319822; Smithsonian photo 61837-A.) 

FIGURE 11.—Balloon meteorograph 

designed by Lawrence Dines, 1929. 

This represents the final development of design approach first introduced by William Dines in the early 

1900s. (From W. Middleton and A. 

Spilhaus, Meteorological Instruments, 1953, fig. 166.) 

than Millikan had hoped for—a result of the failure of rubber 

manufacturers to achieve the quality of the pre-war Russian 

balloons. 

Rubber manufacturers in Europe and the United States did 

provide balloons that were quite satisfactory for the more modest altitude ranges required by most meteorological researchers. Moreover, the use of such meteorological balloonsondes 

increased after World War I because using kites and captive 

balloons for obtaining routine meteorological data had become 

more difficult after about 1910, as a result of the proliferation 

of electric power lines in Europe and in the United States. Accidents resulting from the contact of the wire cables of captive 

balloons and kites with electric power lines demonstrated the 

dangers inherent in the use of tethered vehicles. Thus, although 

captive balloons were used throughout World War I, their use 

was limited to less populated regions after the war. 

The availability of airplanes as instrument platforms compensated somewhat for the decline in the use of kites and captive balloons, as did the more frequent use of balloonsondes to 

obtain routine meteorological data. Thus, in the 1920s the increased use of balloonsondes prompted renewed efforts to design lighter and cheaper payloads (Middleton, 1969b:98). 

After World War I, a number of balloonsondes, with and 

without clockwork-driven recorders, were produced that emphasized simplicity and low weight. An instrument (Figure 10) 

designed by American scientist S.P. Fergusson exemplified the 

American balloonsonde of that era (Middleton, 1969a:99). Its 

clockwork-driven recorder, however, proved to be too complicated. More successful designs were based upon the concept of 

Dines, which used the small displacements of the sensors themselves to scribe a record of temperature and pressure. This ver-
NUMBER 53 13 

FIGURE 12.—Jaumotte meteorograph of 1931. This is an example of a European instrument that was strongly 

influenced by the Dines design. (Instrument in NMAH collection, catalog number MHT 319823; Smithsonian 

photo 48467-E.) 

sion strongly influenced the design of meteorological balloonsondes from the early 1900s through the 1930s (Middleton and 

Spilhaus, 1953:231). 

In the late 1920s, Dines' original design was modified by his 

son Lawrence (who, like his distinguished father, became a 

well-known meteorologist) to include a hygrometer. The instrument (Figure 11) measured temperature, pressure, and humidity. The device was mounted inside a light bamboo frame that 

provided great shock absorption and was so rugged that the 

parachute was eliminated. The entire unit, including frame, 

weighed only 100 grams and was capable of being lofted by a 

small (350 gram) rubber balloon (Middleton and Spilhaus, 

1953). This probably represented the most sophisticated design 

of a lightweight, low-cost, nontelemetering balloonsonde for 

obtaining synoptic meteorological data. Dines' 1929 meteorograph had features that influenced the subsequent designs of 

most European balloonsonde meteorographs (Middleton, 

1969a: 100). For example, a unit designed by French meteorologist Jean Jaumotte (Figure 12) is an example of a related design. 

In summary, by 1929, the technology of unmanned sounding 

balloons had developed to a point that made the balloonsonde 

the most effective tool for studying the upper reaches of the atmosphere. Until the advent of the World War II rocket, there 

was no other vehicle capable of reaching 30-kilometer heights. 

Moreover, the Baume-Pluvinel, Abbot, and Millikan balloonsondes conclusively showed that important geophysical information could be obtained, using complex payloads of sophisticated design. On the other hand, the success of the Dines-type 

instrument, coupled with small rubber balloons, demonstrated 

that the balloonsonde also was of meteorological value for the 

collection of synoptic weather data at lower altitudes. 
14 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

The major problem of the balloonsonde, in both high-altitude 

geophysical research and in the collection of meteorological 

data at low altitudes, was payload recovery. The probability of 

recovery was low in some unpopulated locations (such as the 

Arctic), and the loss of the data from one-of-a-kind payloads in 

special research flights was very costly. For relatively inexpensive meteorological balloonsondes that were launched in quantity, the loss of the data from an occasional lost payload was not 

serious, but the considerable lapse of time between the flight 

and recovery of the data significantly reduced the usefulness of 

balloonsondes in weather forecasting. 

In the late 1920s, the advent of the radiosonde was a "natural" development of the balloonsonde whose evolution was determined mostly by advances in radio technology; that is, the 

necessary theoretical groundwork for the radiosonde existed 

long before such an instrument was produced. However, the 

possibility of linking balloon-borne sensors to a ground station 

via radio depended upon the development of transmitter components of suitable size, weight, power, and ability to survive 

the temperatures and pressures to which balloon payloads were 

subjected. Indeed, the first radiosondes utilized principles of 

telemetering by wire that had been understood more than three 

quarters of a century earlier, and they embodied principles of 

radio that had been well understood for almost two decades. 
2. Preradio Telemetering, 1842-1894 

The development of the electromagnetic telegraph progressed rapidly after about 1830, and by 1840 many miles of 

circuits had been built. At that time it was apparent to scientists 

that there were at least two ways in which the telegraph could 

be applied to problems of meteorological data gathering. First, 

data already recorded could be transmitted very quickly to centers for analysis and archival storage. Second, and more intriguing, the instruments themselves could be connected by 

telegraph wire to deliver their data directly to the place where it 

would be used. Both applications came into use fairly quickly, 

but the second approach presented difficulties (Borden and 

Thynell, 1948). 

Prior to 1840, information transmitted by telegraph had to 

originate in a discretely coded or symbolic form. In normal 

(nonmeteorological) message transmission, this form was standard language, including numerals and punctuation marks. Different schemes of representing alphabets and numerals were 

available, such as the Morse-Vail code, but these were only 

translations of the original symbols. They were, nevertheless, 

still discrete symbols and were easily sent over the telegraph as 

such. 

Meteorological data presented a fundamentally different requirement. Data from the thermometer, barometer, and windregistering devices were not discrete but, rather, were continuous or analog in form. Scratching this data on a smoked drum 

or inking it on paper was simple as long as the drum or paper 

was in direct contact with the sensing device itself. Indeed, 

these, or closely allied methods, were the universal recording 

system for all early meteorographs. Transmitting such analog 

data to a remote location, without an observer to first "read" 

the data and convert it to discrete symbols, was a very difficult 

problem. 

This problem first became apparent with the attempt to send 

graphic or pictoral images over the telegraph. Although these 

images were not considered "data," the problem was the same. 

Part of the solution was already at hand. A marking scheme for 

symbols had been developed in 1828 that used an electrochemical indicator connected to an electrostatic telegraph. The missing link, a way to produce simultaneous and continuous marking at both ends of a transmission path, was worked out and 

patented in 1842 by an English physicist, Alexander Bain. 

Bain's patent incorporated the electrochemical marking system 

and brilliantly adapted a time-synchronization scheme developed by French scientist Claude Chappe in order to preserve 

the geometric integrity of the graphical data (Garratt, 1965). 

The system used pendulums at both ends of the transmission 

path as shown in Bain's 1842 patent (Figure 13) (Ranger, 

1926:2). 

A pendulum at the receiver (shown at right in Figure 13) had 

a slightly longer natural period than a similar pendulum at the 

sending station (shown at the left). The sending pendulum 

closed a circuit at one extreme of its swing and energized an 

electromagnet through the telegraph lines to capture the receiving pendulum near the end of its swing. The receiving pendulum could not swing past its starting position before the electromagnet captured it. If it did, synchronization with the 

sending pendulum, which was intended to release both simultaneously from their starting positions, would be lost. When the 

sending pendulum reversed direction to start another swing, its 

contact opened and the receiving pendulum began in perfect 

synchronization. The slight difference in natural period had no 

effect on the average period of the receiving pendulum because 

synchronization was reestablished every cycle. 

Bain's facsimile was probably the earliest version of analog 

data-transmission techniques. It clearly provided a prototype 

for telemeteorographs capable of directly sending analog data 

and lacked only radio to be the prototype of an analog radiosonde. 

After 1837, Charles Wheatstone in England concentrated on 

improving the electromagnetic mechanisms used in the telegraph system. His ideas subsequently influenced the development of meteorological instruments (Garratt, 1965). Particularly impressive was a system derived from Chappe's time 

coding, which removed most of the burden from the operator 

for accurate interpretation of the telegraph symbols. It consisted of a continuously stepping disc at the sending station 

synchronized with a similar disc at the receiver. The disc was 

made to pause at the position of symbols to be recorded. The 

improvement over Chappe's device was that these discs were 

synchronized over the two-wire circuit. 

In 1843, Wheatstone introduced the first "telemeteorograph," a device that transmitted overland by wire the readings of a barometer and a thermometer to a receiving station 

many miles from the sensors (Middleton, 1969a:317). Moreover, the possibility of transmitting such meteorological data 

from kites or balloons also was investigated. In the same year, 

Wheatstone devised a telemetering thermometer for use with a 

captive balloon. The electrical output of the thermometer was 

connected to the ground by the balloon's tether, which incorporated two copper wires that transmitted the signals to a 

land-based receiver. Interestingly, this concept was abandoned 

and was not tried again until 1917. The "immediacy" of the 

data obtained in this way probably was perceived as a negligi15 
16 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

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FIGURE 13.—Alexander Bain's two-pendulum concept. This is a reproduction of the diagram in Bain's 1842 

patent. (From R. Ranger, Proceedings of the Institute of Radio Engineers, volume 14, 1926.) 

ble improvement because tethered kites and balloons carrying 

self-recording instruments could be cranked down within minutes of reaching their maximum altitude, thus allowing the 

timely retrieval of data. The use of overland telegraphy links to 

sensors in remote locations also was developed in the midnineteenth century. 

About 1868, a Dutch professor, F. van Rysselberghe, developed a meteorograph, using the earlier ideas of Wheatstone and 

others. It converted readings of pressure, temperature, and 

wind speed to markings on a rotating drum. In 1874, another 

Dutch scientist, E.H. von Baumhauer, published a design suitable for telemeteorography from a balloon or kite, similar to 

van Rysselberghe's, which also produced markings on a 

smoothly rotating drum (Middleton, 1969a:318-321). Transmission of markings between similar drums required near synchronization of rotation, and von Baumhauer proposed the use 

of a signal over telegraph wire to start them simultaneously. 

Subsequent synchronization relied upon constant speed of the 

individual drums; an "open-loop" method identical in principle 

to the electrostatic telegraph system that Chappe had devised 

almost a hundred years earlier. 

A deficiency of von Baumhauer's telemeteorograph was that 

the rotation speed of one clock- or motor-driven drum could 

not be made perfectly equal to another, given the technology of 

that time. Thus the receiving drum would eventually drift out 

of step with the transmitter. Working to improve performance 

of the telemeteorograph, H. OUand, a Dutch instrument maker, 

built an elaborate system in 1877, using a sensor design similar 

to von Baumhauer's drum and incorporating a pendulum synchronization system identical in principle to Bain's (Middleton, 

1969a:324). Periodic synchronization of the rotating drums 

greatly shortened the period during which they were required 

to maintain constant speed and thus allowed satisfactory operation for an indefinite period. Olland personally knew von 

Baumhauer and, at his suggestion, used some of his ideas. It is 

unclear, however, if Olland was familiar with Bain's work, so 

he may have "reinvented" Bain's two-pendulum concept. The 

overall scheme became known as the "Olland Cycle" (Middleton, 1969a:324) as used in later instruments, including radiosondes, but it clearly incorporated ideas of Bain, von Baumhauer, and van Rysselberghe. 

This principle is extremely effective and inherently simple. It 

is so attractive that it has been used, in appropriate technology, 
NUMBER 53 17 

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(From German Patent Number 93032, 1896.) 
li SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

by virtually every development in data communications up to 

the present.4 The concept, reduced to its most basic form, involves the entrainment of one oscillator by another, where the 

degree of stability of the chosen oscillators is sufficient for the 

purpose at hand. In communications, unsynchronized timekeeping devices with stable, short-term periods are used at 

sending and receiving ends of a path. The sending timekeeper 

resets the receiving timekeeper periodically by some means to 

a specific phase point to achieve long-term synchronization. 

A modern example is the mechanical teletype developed for 

text transmission, which evolved into a universal system of 

asynchronous data transmission between computers. In these 

systems, a signal is transmitted instructing the receiver to begin 

recognizing a prearranged number of elements (bits) of a fixedlength message at a predetermined rate. After the prearranged 

number of elements, the receiver waits for a signal indicating 

that the sending interval is over, then waits for a new start signal and the cycle repeats. 

The only other synchronization scheme in common use for 

data telemetry is one in which timing information is contained 

within the overall symbol stream. Timing information modulates the transmitted signal orthogonally to the data so that both 

can be separated at the receiver. This technique originated after 

1940 and did not come into use with meteorological data until 

the beginning of rocket telemetry. It has never seen significant 

use in mass-produced balloon-borne radiosondes. 

A significant deficiency of the van Rysselberghe-Olland systems, as well as any derivative of the Bain facsimile, was that 

they demanded clockwork motions of virtually uniform speeds 

during the period of each observation. In attempting to alleviate 

this problem, an ingenious system was invented and patented in 

1896 by two Swiss scientists, Luigi Cerebotini and Albert Silbermann. This system established a fundamentally new method 

of information transmission that converts the magnitude of a 

continuous scalar variable into a digital value and then transmits the value, using discrete symbols. The accuracy with 

which the original value can be transmitted is not at all limited 

by the method of transmission, given sufficient time. The accuracy of analog data, on the other hand, is closely linked to the 

parameters of the transmission system. Analog-to-digital conversion of data before transmission is virtually universal in 

modern telemetry for the purpose of maintaining high accuracy. 

In their device (Figure 14), Cerebotini and Silbermann incorporated a number of electrical conducting bars inset in a drum 

of insulating material. As the drum rotated, the arm of a meteorological sensor rested against it. The number of bars contacted 

per revolution depended upon the position of the arm, hence it 

was necessary only to count the "on-off' contacts at the other 

end of the telegraph line to obtain the reading of the meteorological sensor activating the arm. The transmission was "digital," and accuracy was thus independent of the uniformity of 

the rotation speed of the drum (although it was limited by the 

number of conducting bars incorporated in the drum). This 

method was subsequently adopted by French scientist, Robert 

Bureau, in constructing his first radiosonde (see pages 31-33). 

As the technology of radiosondes developed, however, there 

was a strong prejudice in favor of analog systems, which had 

been the basis of meteorographs for more than one hundred 

years. The usual method for data display was a two-dimensional graph, and it was difficult for instrument designers to 

break away from a mechanism that operated in the same way, 

even though the advantages of digital coding were obvious. 

Consequently, there was a mixture of analog and digital representations of data as the radiosonde evolved. 

By the turn of the century, interest in overland telemetering 

of meteorological data from remote stations had waned; however, the basic principles of telemetering, embodied in nineteenth-century telemeteorographs, were the source of ideas that 

scientists used to develop the radiosonde. These ideas, coupled 

with the developments in radio, made the radiosonde possible. 
3. Radio Telemetry, 1895-1929 

By 1850, the telegraph had become firmly established as a 

means of transmission of language and data; however, there already was frustration with the difficulty and expense of installing wires. It had been known since the work of Faraday and 

Henry that electric currents could produce effects in a circuit 

not connected by wires to the source of the current. The phenomenon, called mutual induction, had fascinated experimenters in Europe and America as a possibility for creating a wireless version of the telegraph. The range of induction in the 

1850s was disappointing, however, only a few meters. Although induction transmission was extended to a few kilometers by the 1890s, it still did not offer a workable means of 

wireless communication (Blake, 1928:282-284). 

Between 1865 and 1886, many experimenters demonstrated 

the existence of propagating electromagnetic radiation. A great 

deal of this work was unpublished or was reported in popular 

literature, rather than in scientific journals. A few articles described results that probably were caused by radiation, but the 

phenomenon was poorly understood and was not well explained by the investigators (Thomson, 1853). 

The early confusion between induction and radiation is not 

difficult to understand. Electromagnetic induction is related to 

radiation, as explained by a theory advanced by James Clerk 

Maxwell (Maxwell, 1865). However, Maxwell's theory remained beyond the ken of most investigators until Heinrich 

Hertz, who, working with a firm understanding of mathematics 

and Maxwell's equations, first conducted energy from a spark 

discharge by wire to a metal loop containing a small gap that 

would spark after the discharge (Hertz, 1888). Hertz then set up 

an identical loop at a distance and observed that it sparked at 

the same time. He subsequently proved the wave nature of this 

interaction by carefully demonstrating reflection, refraction, interference, and velocity. Hertz correctly showed that the wavelength was about 25 centimeters. Although his maximum range 

of transmission was only 19 meters, this was more than 50 

wavelengths. The range of transmission, expressed in wavelengths, was more important than just an extension of communication distance. It proved that the signals were indeed radiation and not caused by the induction effect. Maxwell's 

equations show that the electric field at a distance from a conducted current can have components proportional to wavelength divided by distance, distance squared, and distance 

cubed. Only the component inversely proportional to distance 

is associated with self-propagating radiation; the others produce induction effects. By a distance of 50 wavelengths from 

the source, the inverse squared and cubed terms will have essentially disappeared, leaving only the true radiation. 

Maxwell's radiation became known in the literature as Hertzian Waves. Experimenters duplicated the work and extended 

the experiments across greater distances for application to 

wireless communication. The prospect of a lucrative business 

in making apparatus to replace wire telegraphy interested both 

skilled scientists and tinkerers (Blake, 1928). Individuals who 

used various techniques and who achieved successful results in 

the period 1887 to 1890 included Edouard Branly, Alexander 

Popov, Gugliemo Marconi, Nicholas Tesla, Ferdinand Braun, 

and Sir Oliver Lodge. By 1895, Marconi had shown conclusively that a practical system of wireless signalling was possible. By 1896, Popov and Marconi had demonstrated wireless 

systems with a range of about 10 kilometers. The next year 

Marconi, who sought commercial success, had reached a range 

of 70 kilometers. (In 1909, Marconi and Braun, whose "sparkless antenna system" represented a significant improvement, 

shared a Nobel prize for the development of wireless telegraphy.) 

At the turn of the century, wire telegraphy spanned thousands 

of miles in Europe and America. Wireless offered the possibility of communication across long distances without the costly 

networks of wire. This was a compelling goal, and enormous 

amounts of money and effort were dedicated to its accomplishment. There was a rapid progression to large, powerful apparatus that seemed to be the means of obtaining greater range. Use 

of such huge equipment to return meteorological data from 

aloft appeared not only impractical but foolish, so the transition 

from balloonsondes to radiosondes was not seriously considered at that time. 

The brute-force style of transmission apparatus was confirmed by contemporary quantitative theory. Large antennas 

created higher-amplitude radiation fields and consequently 

larger signals at the receiver. The wavelength at which these 

large antennas absorbed energy (from impulse or spark excitation) and radiated efficiently was proportional to their dimensions.5

 Consequently, long wavelengths were necessary for 

communications, on the order of the antenna dimensions. Empirical studies undertaken by Hertz (1888) showed that range 

was proportional to power and antenna size, as predicted by 

Maxwell's theory. Absorption of the waves increased with distance and appeared to become virtually complete below about 

200 meters, an added obstacle to shorter wavelengths and 

smaller, lighter equipment. 

In the years after Hertz's work, a major obstacle to greater 

range, in addition to power and propagation, was the sensitivity 

of the receiving apparatus. At the outset it was obvious that a 

receiver that depended upon the display of sparks was imprac19 
20 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

tical for use at distances needed for communication. A solution 

was at hand, however, even before Hertz's work, in a scheme 

used by Hughes for induction receiving in the 1870s (Hughes, 

1878). He had utilized a receiving circuit that passed current 

through an assembly of carbon powder originally developed as 

a sound-to-current transducer for the emerging technology of 

sound transmission over telegraph wires. Various experimenters (see Howeth, 1963:15-23) had demonstrated properties of 

this type of device, consisting of an evacuated tube of metal filings between silver electrodes that would serve as a sensitive 

detector for wireless radiation. Called the "coherer," it was perfected by two American engineers, Forbes and Lodge, between 

1892 and 1894 (Lodge, 1908:32-56). This device quickly became the standard wireless detector, with virtually no competition until the invention of Fessenden's electrolytic detector in 

1903. 

Fundamental improvements were made in wireless technology that extended transmission range (but did nothing to reduce the size of the required equipment) between 1897 and 

1900. In order to improve conversion of power into radiation, 

Lodge used inductance and capacitance to make the antenna 

circuit resonant at the wavelength desired for transmission. In 

addition to improving efficiency by better matching of impedance between the antenna and generator, this substantially narrowed the range of wavelengths in the radiation. When Marconi applied similar tuning to the antenna circuit of a receiver, 

the overall effect was dramatic. He achieved the first transatlantic contact on 12 December 1901, at about 1000 meters 

wavelength. Further progress in narrowing the bandwidth of 

transmissions followed shortly, with development of the 

Poulsen arc and the Fessenden-Alexanderson alternators. Instead of the relatively inefficient process of generating a wide 

band of wavelengths by spark discharge and selecting only part 

of this spectrum by a resonant circuit, these devices generated 

oscillations more-or-less in the wavelength range desired. An 

additional reason for improved efficiency was that radio-frequency energy generated by spark transmitters was discontinuous, whereas alternators generated continuous undamped 

waves. The time-averaged power available for transmitting was 

therefore greater. Reactive parameters of the antenna circuit 

still influenced efficiency of energy conversion to radiation, 

but wavelength was fairly well defined and could be selected 

for favorable propagation (Morecroft, 1921:511-513). 

In 1904, the first developments occurred that would lead to 

the construction of small radio transmitters that operated at 

wavelengths suitable for meteorological data transmission 

from balloon-borne instruments. The beginning, however, lies 

not with transmission but with reception. J.A. Fleming, who 

earlier had worked with Marconi to establish transatlantic wireless, used the "Edison effect" to achieve high sensitivity as a 

detector, replacing Branly's coherer. The Edison effect, which 

Edison had reported in 1883, refers to the phenomenon of electric current flow through an evacuated tube containing electrodes. By 1901, physicists understood this effect, and Fleming 

used it to "rectify" or convert the radio frequencies picked up 

by a receiver to direct current in a headphone or galvanometer 

(Chaffee, 1933:1-16). 

Fleming's achievement quickly led other investigators to use 

these "vacuum tubes." In 1906, Lee de Forest showed that by 

adding a third electrode, current flow between the other two 

electrodes could be controlled, using virtually no power (de 

Forest, 1906). Still, probably as a result of Fleming's lead, 

early work on application of these tubes was concentrated on 

improving the sensitivity of radio reception. Unintentionally, 

this approach was fortunate for eventual application to meteorological data trans-mission because it fostered development of 

small, low-powered circuits before the obsession with high-energy transmission could dominate the entire effort of engineers. 

In fact, much of the early work with tubes, called "audions," 

was devoted to amplification of telephone communications 

over wire; also a comparatively low-power application (de Forest, 1906). 

From 1905 to 1915, progress in radio technology was steady 

but not spectacular. After 1915, the demands of the military 

services fighting in World War I gave impetus to the development of more efficient tubes and circuits. For example, in 1916, 

the first use of aircraft radios had occurred (Figure 15), although the radios still were too large to be adapted for balloonsonde use (Tyne, 1977:117). There was intense competition for 

financial rights regarding inventions within the emerging business of radio, and patent litigation was common. Not until the 

latter part of that period did developments in applied technology reach a point that promised help for scientists attempting to 

send data directly from a balloon carrying meteorological instruments. 

There was no lack of interest in this application. German science historian Paul Beelitz (1954:67) reported a private communication from E.F. Herath, written in 1950, indicating that 

Herath and other German meteorologists, Max Robitzsch and 

Alfred Wegner, considered using balloon-borne radio transmitters in the summer of 1914. The vacuum tubes that were available, however, such as the de Forest audions, were poorly evacuated, which rendered them unpredictable in circuits. (Two 

American engineers, Langmuir and Arnold, succeeded in 

building much-improved high-vacuum tubes in 1912-1913 at 

the General Electric and AT&T laboratories, but they did not 

reach production for several years (Tyne, 1977:133-157)). This 

defect, together with fragile construction, made vacuum tubes 

unsuitable for service in the relatively hostile environment to 

which balloons ascend. Another difficulty was that, in 1914, 

the only circuit design that could make a balloon-borne transmitter practical, the continuous wave oscillator, was still in its 

infancy and was not widely known. 

This situation did not deter experimentation with radio-frequency energy to get data from balloons aloft. Pierre Idrac in 

France, and Herath and Robitzsch in Germany, in separate experiments during 1917, used radio-circuit techniques to send 

data down the wire of a tethered kite. No doubt some slight ra-
NUMBER 53 21 

* 

1916 

FIGURE 15.—Gugliemo Marconi's air-to-ground radio, circa 1916. (From a Marconi Instrument Company advertising brochure mailed to the electronics industry in the spring of 1987.) 

diation did occur in the process, but no attempt to measure this 

was recorded (Middleton, 1969a:276). 

Early in 1917, Herath and Robitzsch produced a device in 

which a contact on a revolving drum touched a stylus of the 

sensor and energized the primary of a small spark coil (Middleton, 1969a: 103). The secondary of this coil was paralleled with 

a capacitor and spark gap. One side of this broadly tuned circuit was connected to the steel tether of the kite. Detection was 

achieved by a telephone receiver connected to the kite. Data 

coding was analog by time interval. The stylus-drum contact 

was closed once each revolution of the drum at a fixed point on 

the drum, regardless of stylus position. It was closed again at a 

time dependent on the position of the stylus, thereby converting 

stylus position into a time interval. As long as the signal from 

both closings was clearly received, the scheme was self-calibrating and precise to about one degree centigrade for temperature readings. 

A more elaborate experiment was conducted in September of 

1917, in France (Rome, 1936:278). Pierre Idrac used a self-interrupting spark gap, or "buzzer," in the balloon to excite a parallel-tuned circuit (Figure 16). The buzzer was energized by a 

contact driven from the meteorological sensor. The balloon 

tether again served as an electrical conductor for the radio-frequency signals. Idrac significantly improved reception by using 

a parallel-tuned circuit coupled inductively to the tether at the 

ground. A rectifier and telephone receiver across this tuned circuit completed the receiving circuit. Idrac pointed out an important feature of this scheme that derived from the radio-frequency circuits: by using multiple transmitters tuned to 

different frequencies, the same tether could conduct data simultaneously from several sensors without interference. On the 

ground a tuned receiver could separate these signals. Idrac also 

had a capacitor sensitive to temperature. This would code the 

data into a frequency that could be measured on the ground. 

This technique subsequently was used by some radiosonde developers. 

The research of Herath, Robitzsch, and Idrac anticipated developments in radio technology that would lead to the balloon-borne radio transmission of meteorological data. They already were working with the same radio-frequency energy and 

circuit techniques that drove wireless telegraphy. It was only 

the lack of small, rugged, radio-frequency oscillators that kept 

them from making this innovative transition. 
22 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 16.—Tuned circuit of Pierre Idrac, circa 1917, used in the meteorograph that transmitted sensor data via 

wire to a ground receiver. (From E. Rothe, Comptes Rendus, volume 170, 1920.) 

Although the vacuum tubes available in 1915 were of poor 

quality, a very wide range of applications had been forseen by 

companies interested in manufacturing tubes, and vigorous 

commercial development was begun. It was the three-element 

audion that made possible invention of the continuous-wave 

oscillator. This occurred during the search for more sensitive 

methods for wireless reception. Sometime after 1911, an audion circuit was built with substantial coupling or "feedback" 

from the resonant coil in the plate output to a similarly resonant 

coil in the input grid circuit. When the grid coil also was coupled to an antenna and the feedback was adjusted carefully, it 

proved to be an extremely sensitive detector of wireless transmissions. Adjustment for even more feedback caused the circuit to become unstable and to produce oscillations. 

The circuit, called a regenerative detector, was first described 

to the Institute of Radio Engineers by American engineer Edwin Armstrong in 1913 (Armstrong, 1917). Priority was controversial, however, and the courts later credited de Forest with 

originating the concept. Equally as important as its receiving 

virtues was the circuit's property of self-oscillation. The usefulness of this property probably was first recognized by Meissner, who received a German patent for an oscillator circuit in 

1913. However, the priority of this circuit also was disputed between Langmuir, Meissner, Armstrong, and de Forest, who 

again turned to the courts for resolution. In spite of these arguments, a means finally was available for producing sustained, 

high-frequency, oscillating currents for radio transmission 

without the great size and inefficiency of the Poulsen arc or the 

Alexanderson alternator. The circuit and vacuum tube did require optimization for producing high-frequency output as a 

transmitter, but these developments soon took place. In 1913, 

Meissner demonstrated the use of a vacuum-tube oscillator for 

radiotelephony between Berlin and Nauen (Meissner, 1922). 

The gas-filled tube used in this experiment produced 10 watts 

of radio frequency (RF) output. By 1912, Langmuir and Arnold 

already had produced the high-vacuum tubes necessary for stable operation over a wide range of power, and the continuouswave (CW) oscillator began to dominate all other methods of 

radio-power generation. By 1915, vacuum-tube oscillators 

were being used by AT&T and Bell broadcasting; one Bell application used 500 tubes in parallel (Craft and Colpitts, 

1919:310). 

It is important to trace the evolution of the CW oscillator because, even in its most rudimentary form, it is quite suitable as 

a radiosonde transmitter (Goldsmith, 1918). Due to size and 

weight limitations imposed by the balloon environment, radiosonde CW circuits changed very little from the earliest design 

to the mass-produced radiosondes of the 1940s and 1950s. This 

circuit, a plate-tuned triode oscillator with inductively coupled 

feedback to the grid, was used in the earliest regenerative receivers (Figure 17). Its adaptation to radiosonde service (Figure 

18) required only the attention of meteorological scientists and 

sufficiently rugged high-vacuum tubes. A final consideration— 

storage batteries with adequate capacity to power the tube filaments and plate circuit at the low temperatures aloft—was important but not critical to the genesis of a true radiosonde. Batteries large enough to overcome low efficiency could be carried 

by using larger balloons. 

After 1900, the work done by radio engineers was augmented by amateurs or hobbyists in Europe and America (Ster-
NUMBER 53 23 

V 

FIGURE 17.—Circuit by H.J. Round for using a three-electrode valve for beat 

reception, circa 1913. (From J. Fleming, The Thermionic Valve and Its Development in Radiography and Telegraphy, London, 1919, fig. 112.) 

ling, 1928). These enthusiastic individuals with, for the most 

part, no commercial motives, were highly innovative and did 

not have the obsession with high power and long wavelength 

that characterized "professional" radio development. It is essential to include amateur radio technology in the history of radiosonde evolution because it brought about the development 

and use of low-power transmitting oscillators. In 1917, President Wilson ordered all amateur wireless stations dismantled 

and all commercial stations turned over to the U.S. Navy. The 

policy did not, however, halt the hobbyist's valuable contributions to radio science because many took up similar work in 

government laboratories. Another large group of amateurs contributed to the war effort by serving as wireless operators. 

The development of vacuum tubes suitable for operation of a 

triode CW oscillator as a balloon-borne radiosonde occurred 

Cs 

•ir-r 

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W¥\^ 

— - H • • * 

AC£ 

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FIGURE 18.—Tuned plate oscillator circuit used in radiosondes, circa 1934. LI 

and L2 are coupled to provide feedback for oscillation. Note the similarity to 

the circuit shown in Figure 17. (From W. Wenstrom, Monthly Weather Review, 

volume 62, 1934.) 

between 1914 and 1918, although the secrecy surrounding such 

work in World War I obscured the record of developments in 

vacuum tubes and low-power transmitting oscillators from 

1917 through about 1919. Development of vacuum tubes was 

both retarded by wartime secrecy and, at least regionally, advanced by wartime need for improved field communications. 

Low-power, battery-operated transmitters, using new, smaller, 

vacuum tubes, were built for military signaling, and descriptions of them appeared in post-war literature. For example, 

Meissner (1922:3) described a high-vacuum tube transmitter 

with 10- to 15-watt output, built by the Telefunken Company in 

1915, and indicated that it was "practically the same arrangement as the transmitter used experimentally by the Allies at the 

end of 1917 on the West Front." A French "R" type high-vacuum tube was patented in 1916 and used in Allied receivers. 

The filament requirements were 3.7 volts at 500 milliamps, and 

75 volts was adequate for the plate supply. These figures were 

compatible with the battery power supply for a field CW transmitter. The United States began to develop two-way radiotelephone sets shortly after its entry into the war for use on submarine chasers and airplanes. These sets used standardized highvacuum, oxide-coated, filament tubes and had a simple, fieldserviceable design (Tyne, 1977:190-278; Blake, 1928:244-

245). Large-scale production of this apparatus was not 

achieved in time for significant field use in airplanes by the 

U.S. Army. 

Wartime need for small, portable equipment led to interest in 

shorter wavelengths. The initial reason was that smaller antennas would be more practical and easier to conceal in the field. 

Smaller antennas, however, required shorter wavelengths to be 

most efficient. This was a fortuitous development for balloon-borne transmitters because the few hundred meter wavelength of common radiotelegraphy was impractical for a balloon antenna. Initial experiments with shorter wavelengths 

brought an unexpected result. It was discovered that range 
24 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 19 (right).—Catalog and instruction book for building an apparatus for 

continuous wave operation, circa 1921. (From Anon., The Radio Industry, New 

York, 1928, fig. 3.) 

FIGURE 20 (below).—Circuit for using a three-electrode Thermion as a generator of oscillations, circa 1919. Coils R and L in the grid and the plate circuits 

are closely coupled. This circuit, which has remained virtually unchanged, and 

a Hartley circuit, where the grid is tapped off the plate coil, later became standard radiosonde transmitters. (From J. Fleming, The Thermionic Valve and Its 

Development in Radiography and Telegraphy, London, 1919, fig. 137.) 

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could be increased substantially during daylight hours, to about 

the same range as nighttime. In 1916, Marconi made transmissions across more than 1600 kilometers in Europe, on a wavelength of 32 meters. He characterized these as "beam" transmissions because the radiation was directed in a single 

direction by placing reflecting structures near the antenna 

(Weagaut and Waterman, 1918). Even shorter wavelengths (in 

the few meter range), called "ultra-short," were produced by 

numerous experimenters (Wenstrom, 1932). The benefits of 

these short wavelengths were confirmed after the war by Franklin in England, Conrad in the United States, and by many amateurs on both continents. Early radiosonde prototypes were 

greatly improved through the use of shorter wavelengths. 

In late 1918, references to CW oscillator and miniature vacuum-tube development again appeared in the literature. Availability was a problem, especially to amateurs, because the wartime control of wireless operation and production was not 

rescinded immediately after the armistice. Authors speculated 

about when amateurs would have access again to the new vacuum tubes and suggested that such tubes would be of considerable use in transmitters. Civilian rights to radio, amateur and 

commercial, were restored by the Government in 1919. Soon 

there were many developments in radio science, both basic and 

applied (Espenscheid, 1922). There is no question that suitable 

circuit designs, as well as components for construction of a viable balloon-borne radio transmitter that could be modulated by 

meteorological sensors, existed by 1920. RCA was producing 

thousands of triode vacuum tubes of types UV200, UV201, and 

UV202, with an improved, oxide-coated, tungsten filament that 

required one ampere at five volts. Also, the de Forest company 

was still producing audions, mostly for amateur use (Tyne, 

1977:300-340). (In 1922, the UV201 was replaced by the type 

UV201A, requiring only 250 milliamperes of filament current, 

which had been developed by Western Electric for the U.S. 

Navy in 1920. Sixty-milliampere varieties followed shortly 

(Tyne, 1977:300-340).) The cover of a 1921 RCA parts catalog 

and instructional manual featured apparatus for the construction of CW transmitting gear (Figure 19). Amateur designs for 

small, one-tube transmitters proliferated. An example from a 

1920 journal is a single type VT-1 tube in a Hartley-type oscillator operated from dry batteries that provided 50 volts platesupply potential. (Basic oscillator types are named usually for 

the method of coupling between plate and grid circuits; the 

Hartley oscillator is inductively coupled.) Similar designs were 

used by amateur radio enthusiasts in Germany and France. This 

transmitter could be on-off keyed for telegraph code or plate 

modulated for telephony. It produced a few watts output operating on 300 meters (Figure 20). It could be placed into a balloon and on-off keyed by a sensor, such as Idrac's revolving 

drum, to make a radiosonde. 

We did not, however, find any published record of such a 

complete and advanced development of radio transmitters for 

balloon use until 1927. Why this was so is a curiosity. The pioneers of radiosonde development, Blair, Bureau, Duckert, and 

Moltchanoff, wrote after 1927 that they had begun work on the 

development of radio transmitters for use with free balloons in 

the early 1920s, but the results of their research did not appear 

at that time in professional journals (see Section II, "The Early 

Years"). It seems unlikely that the development of a successful 

radiosonde would have gone unmentioned because Soviet and 

German scientists, especially, later emphasized the priority of 

their work. 

A likely explanation for this silence lies in the fact that adaptation of radio technology to the balloon environment was not 

easy, and scientists were not highly skilled in such adaptation. 

Also, as in most technological developments, the successful radiosondes of the early 1930s were the product of evolution in 

design concepts. Thus, development progressed from feasibility experiments in the early 1920s through rudimentary prototypes to practical balloon-borne radio transmitters, with most 

of the defects removed by 1927 to 1928. Developers of the 

early radiosonde were more interested in creating a new technology for obtaining upper-atmospheric measurements than in 

doing atmospheric research per se. The extent to which the radiosonde developers were able to utilize directly the relatively 

mature radio technology of the period from 1920 to 1929 is 

very difficult to determine. It is possible, however, to trace this 

evolution from the fragmentary evidence in the literature. 
II. The Radiosonde: Development and Application 

4. The Early Years, 1921-1928 

As is usual after a major war, the former World War I combatants undertook research to resolve military problems that 

had been exposed by the harsh light of battle. As part of such 

research, the applications of radiotelemetry to meteorology 

were investigated in Europe and the United States. Research 

was prompted by the burgeoning military need for meteorological data stimulated, primarily, by the increased reliance upon 

airplanes as weapons. More specifically, the military needed an 

all-weather means of obtaining timely information about the 

winds aloft and the atmospheric temperature, pressure, and humidity at various altitudes in order to prepare synoptic weather 

maps. The war had revealed that such data were indispensable 

to a fully operational air force, and, therefore, research on improved means for obtaining these data was sponsored by the 

governments of Germany, the Soviet Union, France, and the 

United States. 

In Germany, research on balloon-borne radio transmitters 

was begun in 1921 at the Lindenberg Observatory under the supervision of its director, H. Hergesell. Later, Hergesell 

(1932:50) wrote that such experiments were begun "long before the researches of foreign, and especially Russian, scientists." This appears to be an overstatement, although such research in other countries probably began after 1921. In 1923, 

both the United States and the Soviet Union initiated similar investigations; in 1925, France followed suit. 

Between 1921 and 1928, there was an adaptation of spark 

technology and then of vacuum-tube technology for radio transmission from free balloons. These investigations primarily were 

to prove the feasibility of making a transmitter work in the balloon environment. Some early investigators sought a means of 

tracking balloons aloft, in order to chart wind vectors. This application did not require any modulation of a radio signal, simply the signal itself. Indeed, the use of a radio signal to track a 

balloon to obtain information about wind velocity and direction 

as a function of altitude probably was the first application investigated in Germany and the United States (Blair, 1937). 

It is not surprising that this application was one of the first 

investigated. World War I had demonstrated that information 

about the winds aloft was important not only for airplane navigation, but also for accurate artillery fire and chemical warfare. 

(The use of poison gas in World War I had made a deep impression on military doctrine.) Conventional methods of tracking a 

pilot balloon by using theodolites worked well in daylight, 

when visibility was good, but they were difficult to use at night 

(although lanterns were carried by the balloon) or when poor 

CHOKE, 

C I vflUfilr 

a RCCW 

FIGURE 21.—William Blair's first radio-tracking transmitter, circa 1923-1924. 

"Buzzer" contacts in plate circuit ingeniously allowed voltage step-up so that 

the filament battery could also supply the plate. The unit was designed to be 

flown using a tandem-balloon recovery method, described in section 2. (From 

W. Blair and H. Lewis, Proceedings of the Institute of Radio Engineers, volume 19, 1931.) 

weather conditions hampered visual contact. During World 

War I, sonic methods to overcome these difficulties had been 

tried, but these were found wanting, chiefly because of their 

limited range (Blair and Lewis, 1931:1532). Therefore, the use 

26 
NUMBER 53 27 

FIGURE 22.—First transmitter operated in the stratosphere. The quarter-wave antenna measured 10.5 meters in 

length. It was suspended by balloons, and a transmitter hung at its end. With batteries, the entire assembly 

weighed 2.7 kilograms. Power consumption was four watts, and radiated power probably did not exceed one 

watt. From left to right are the filament battery, the vertical-axis keying windmill, and the transmitter (including 

plate batteries, inductances, and tubes). The unit was constructed entirely from commercially available parts. 

(From R. Bureau, La Meteorologie, volume 7, 1931.) 

of balloon-borne radios as an all-weather, night and day means 

to determine wind vectors promised to fulfill an important military need. 

In Germany, E.F. Herath, a meteorologist, who in 1917 was 

the first to use spark transmitters to send data along the tether 

of a captive balloon, began to experiment with wireless transmitters of similar design. In 1921, he placed a transmitter on a 

free balloon and attempted to track it from the ground. The radio signal was detected by the ground receiver, but accurate 

tracking was not achieved (Beelitz, 1954:67). 

Somewhat more successful experiments were conducted in 

1923-1924 by Colonel William Blair at the U.S. Signal Corps 

laboratory at McCook Field, Ohio. Blair utilized a buzzer 

transmitter like that used in 1917 by Idrac, although Idrac's device transmitted signals by wire from a captive balloon. Blair's 

radio transmitter weighed less than one pound and was carried 

aloft on tandem balloons (Figure 21). The balloons were 

tracked successfully by radio signals for about twenty minutes, 

ascending to a height of almost 4 kilometers. 

Blair also began investigating the possibility of using a balloon-borne radio transmitter as a means of obtaining the wind 

velocities and air temperatures as a function of altitude (Blair 

and Lewis, 1931:1534). He had noted that radio transmitter 

characteristics changed because of the effects of temperature 

and other meteorological conditions. These changes then produced frequency changes in the transmitter signal. Instead of 

attempting to design transmitters that were free from these effects, Blair recognized that the effects themselves constituted a 

means of obtaining meteorological data. Blair (1937:2) later 

wrote "This is possible because...a proper selection of [transmitter] materials and design provides an independent response 

to the different meteorological conditions that is quantitatively 

measurable." In other words, the radio transmitter could be 

converted into a radiosonde by using the components of the 

transmitter as a kind of distributed "sensor" that could be calibrated to measure a specific meteorological parameter. According to Blair and Lewis (1931:1534), Blair constructed such a 

balloon-borne radio transmitter for measuring atmospheric 

temperature and made the first upper-air observations by this 

means early in 1924. 

Blair's device constituted a radiosonde, although this terminology was not introduced until 1931. Blair did not continue 

this work on radiotelemetry because in 1924 he was made chief 

meteorologist for the Army's around-the-world flight. After 
28 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 23.—Radio-Tracking transmitter designed by William Blair in 1927, using a CW vacuum-tube oscillator. 

(From Blair and Lewis, Proceedings of the Institute of Radio Engineers, volume 19, 1931, fig. 21.) This design 

was ingenious because it derived filament and plate supply from single 4.5 volt battery supplying about 200 milliamperes. The power supply was a type of self-energized inverter, somewhat like a buzzer. The magnetic field of 

a transformer pulled open contacts in series with the primary and battery, causing sustained vibration of contacts. 

The secondary of the transformer stepped up the primary voltage to supply about 200 volts to the tube plate. The 

plate coil of the oscillator was in the center of the antenna, which formed a center-fed dipole of 25 meters overall 

length. The operating wavelength was about 125 meters. The transmitter weighed 500 grams and was given the 

Signal Corps designation BC-164. 

Blair completed this mission, he was given administrative assignments that occupied him for several years, to the exclusion 

of his research interests. Thus, although Blair's initial results 

were promising, he did not publish them at the time (although 

he described them in a later publication and lecture) (Blair and 

Lewis, 1931; Blair, 1937). Despite the fact that Blair's transmitters were primitive, he demonstrated the feasibility of two 

applications that were among the most important meteorological uses of radio in the ensuing decades. He also showed that 

accurate radio-tracking of a balloon could determine wind vectors, and that it was possible to measure atmospheric parameters, such as air temperature, with a balloon-borne transmitter 

that relayed this information to a ground receiver. With the end 

of Blair's work in 1924, no substantial progress in this field 

was made in the United States for the next three years. 

The next significant advance occurred in France. Pierre Idrac 

and Robert Bureau had begun work at France's National Meteorology Office that, in 1927, led to the first use of a CW oscillator transmission from a balloon. This was an evolutionary advance from discharge-excited radiation, which had been used 

by Herath and by Blair, to radiation produced by a vacuumtube CW oscillator.6 

Idrac and Bureau (1927:691) constructed a vacuum-tube CW 

radio that had a transmission wavelength of 42 meters. The signal was on-off keyed by an air-actuated windmill device. The 

unit (Figure 22) used tandem balloons and was flown on March 

3 and 7 of 1927, reaching an altitude of almost 14 kilometers. 

The radio signal was received by numerous stations at distances up to 500 kilometers. According to Idrac and Bureau 

(1927:692), the purpose of the flight was to "study propagation 

of short waves as a function of the altitude at which they are 
NUMBER 53 29 

emitted." They concluded that their experiments "probably 

constituted the first reception of radio waves emitted in the 

stratosphere." 

About the time Idrac and Bureau published their results, 

Blair resumed his research on radio-tracking balloons at the 

Signal Corps laboratory, Fort Monmouth, New Jersey. In 1927, 

he designed and constructed an oscillator for high efficiency, 

using a type-199 vacuum tube (Blair and Lewis, 1931). The 

transmitter weighed 500 grams and was given the Signal Corps 

designation BC-164 (Figure 23). Such transmitters were flown 

extensively, beginning in early 1928, using clusters of four to 

six theodolite-tracked balloons. These units provided accurate 

determination of wind vectors by radio tracking. The Signal 

Corps developed elaborate ground receivers for general direction finding, the SCR-170 and SCR-173, and these were used 

for the balloon-tracking work that gave bearings to within plus 

or minus one-half degree of optical theodolite readings. 

The Blair transmitter marked the end of a transition period in 

technology. The instruments developed afterwards were, by 

any standard, similar to modern radiosondes, and they began 

the evolution toward present-day meteorological data telemetry. 
5. The Prototypes, 1929-1930 

The previous sections have outlined the development of balloon-borne instrumentation and advances in telegraphy and radio that provided the technological basis for the radiosonde. Indeed, the radiosonde was described earlier in this publication as 

a "natural" extension of the balloonsonde. In order to understand this process more clearly, it is useful to place the advent 

of the radiosonde in the wider context of the evolution of meteorology. 

The development of meteorology as a science in the nineteenth century occurred chiefly in synoptic meteorology, that 

is, in the creation of networks of widely scattered stations that 

could report simultaneous observations to a central bureau on a 

frequent basis. The ability to gather synoptic data played an 

important role in the growth of meteorological theory. The first 

international compilations of synoptic weather charts were 

made in the early 1800s, and in the ensuing years methods for 

obtaining the data for these charts steadily improved with the 

introduction of better measuring instruments and the creation 

of telegraphic networks. Toward the end of the nineteenth century, improvements in the design of kites and balloons made it 

possible to obtain upper-altitude data on a more frequent basis. 

In the first decades of the twentieth century, improvements in 

weather science were attributable partly to advances in theory 

and partly to the increasingly intensive exploration of the upper 

atmosphere. Theoretical advances were made in the early 

1920s by scientists, primarily Vilhelm Bjerknes, who led Scandanavian researchers in establishing a new conceptual foundation for atmospheric science (Friedman, 1982:343). Bjerknes' 

work was supported by experimental data acquired after 1900 

by sounding the upper atmosphere. Balloons were developed 

that allowed balloonsondes to reach altitudes of 20 to 30 kilometers. Aeronautical progress in other areas made it possible to 

routinely use captive balloons and airplanes to make observations at lower altitudes. As the ability to obtain such data on a 

wide scale was enhanced, it became increasingly apparent that 

synoptic charts of temperature, pressure, humidity, and wind 

vectors at various levels of the atmosphere were essential to 

both weather forecasting and to advances in the meteorological 

theory upon which forecasts were based. 

During the period in which this evolution of meteorological 

theory and practice occurred, the demand for accurate weather 

information grew. The military requirement for such information (as described in the previous section) had expanded dramatically during World War I and remained at a high level in 

the postwar years. The need for weather information also expanded in the civilian sectors of industrialized countries, 

prompted by factors such as the more extensive agricultural exploitation of marginal land and the increased use of airplanes to 

transport people and cargo. Therefore, meteorologists realized 

that the need for more and better weather information could be 

satisfied only by an expansion of programs to probe the upper 

atmosphere on a synoptic basis. By the 1920s, it was apparent 

that the serious deficiencies of existing means for obtaining 

data at various altitudes would inhibit the implementation of 

such programs. 

Methods for probing the atmosphere, using kites, captive 

balloons, and airplanes, were capable of providing the timely 

data required in the preparation of synoptic weather charts. A 

major deficiency of these traditional means, however, was that 

none was usable at all times or in all locations. Captive balloons could not be deployed in high winds. Kites, on the other 

hand, were useless when the wind speed was very low. The 

tether necessary for both vehicles was a significant danger to 

airplanes (as attested to by several fatal accidents in World War 

I). Also, kites and captive balloons occasionally broke loose, 

and their dangling steel cables created a hazard, especially 

when they came in contact with electrical power cables. These 

potential dangers relegated the use of tethered vehicles to remote, relatively unpopulated locales. Airplanes were used to 

obtain weather data toward the end of World War I, and in the 

early 1920s self-registering meteorographs were developed 

specifically for use in airplanes, which flew only in good 

weather and generally only during daylight hours. 

The balloonsonde met the need for an atmospheric probe that 

was usable day or night in virtually all weather conditions. 

Moreover, they could be launched from almost any location. 

These important advantages were offset, however, by a significant disadvantage: a delay, usually of several days, before the 

balloonsonde was found and returned to the central bureau. 

This seriously compromised the usefulness of balloonsonde 

data in preparing daily forecasts. Nevertheless, despite this 

problem, low-cost balloonsondes were developed and used extensively during the 1920s to investigate the atmosphere (Middleton and Spilhaus, 1953:231) (see Section 1, "Nontelemetering Balloonsondes: 1892-1929," Figures 11, 12). 

Meteorologists soon realized that the balloonsonde possessed all the characteristics to make it ideal for obtaining synoptic data, if the time-delay problem could be solved inexpensively. 

Using a radio to transmit balloonsonde data to a ground receiver was an obvious solution that had been discussed as early 

as 1914 by German meteorologists Herath and Robitzsch. Indeed, Beelitz (1954:67) quoted a statement by Herath that indicated that Herath's work with Robitzsch in 1917 on the use of 

30 
NUMBER 53 31 

FIGURE 24.—Robert Bureau's earliest experiments used a mechanical arrangement to minimize weight and to cover a wide temperature range. The first 

sonde linked a bimetalic thermometer and an aneroid barometer to a 

short-wave transmitter so that the two readings could be distinguished. The 

linkage was accomplished by using a rotating cylinder that was partially insulating and partly conducting, a more rapidly rotating contact wheel, and a more 

slowly rotating cam. Shown here is the cylinder used in the Bureau radiosonde 

to sequence output of pressure (P), and temperature (T) sensors. The portion of 

the helix marked "C" is conducting, and the remaining portion is non-conducting material. Note that a partially conducting cylinder was used in the transmitter developed by Herath and Robitzch in 1917, although their instrument 

relayed signals by wire rather than by radio propagation. Bureau extended this 

design, first, to provide a way of accurately determining, at the receiver, the 

rate of rotation of the cylinder and, second, to read out more than one variable. 

Cylinder rotation was driven, at first, by a windmill, later by a clockwork 

device operated by the weight of the instrument, and still later by a spring-powered clock. Precise calibration of the rotation rate of the mechanisms was difficult to obtain unless measured directly. Fast keying pulses provided a way to 

achieve this. 

the tether of a captive balloon to conduct signals from a transmitter (described in Section 3) was begun only after they had 

concluded that the available radios were unsuitable for balloon 

use. By the 1920s, however, advances in radio technology had 

altered the situation, and scientists in several countries began to 

explore meteorological applications of radio. By 1924, Blair 

had demonstrated the feasibility of measuring wind vectors and 

temperature as functions of altitude, using balloon-borne radios, and by 1927 Idrac and Bureau had demonstrated radio 

propagation from a transmitter in the stratosphere. 

In summary, by the 1920s the radiosonde had emerged as a 

practical meteorological tool; that is, developments within the 

relevant technologies (push factors), together with increased 

societal needs for accurate weather information (pull factors), 

made the transition from balloonsonde to radiosonde a "natural" next step in the evolution of meteorological instrumentation. This next step, begun by Blair in 1924, was continued in 

France by Bureau, who launched his first radiosonde in 1929 

(Figure 24) (Bureau, 1931). In quick succession, two similar 

•A 

*M-tt 

FIGURE 25.—Diagram of the coupling of a toothed wheel, which produces frequency modulation in a Bureau radiosonde, to other circuit components. This is 

the wheel that keyed the transmitter on and off, not the wheel that imposes 

small frequency modulation. (From P. Beelitz, Radiosonden, Berlin, 1954.) 

From his earlier experiments on radio propagation from ascending balloons, 

Bureau also determined that identification of the received signal was difficult 

on contemporary receiving apparatus. To alleviate this difficulty he devised a 

method of imposing small frequency modulation on the radio signal by means 

of a rotating toothed wheel placed near the plates of the capacitor in the inductance-capacitance (LC) circuit of the transmitter. As the teeth of wheel rotated, 

the change in capacitance caused a slight deviation in frequency that in turn 

caused a warble in the tone received on the ground, making the signal easier to 

pick out from noise and static. 

types of instrument were developed in the Soviet Union and 

Germany (see "The Moltchanoff Radiosonde" and "The Duckert Radiosonde," below). 

These instruments illustrate clearly how design concepts 

were taken from various existing technologies: the sensors 

were derived from those used in balloonsondes; the data processing was based upon ideas borrowed from telegraphy; and 

the radios owed much to short-wave circuits and components 

developed for portable military transmitters. Furthermore, 

these instruments can be considered prototypes in that many of 

their design features were incorporated in radiosondes that 

were later mass-produced. 

THE BUREAU RADIOSONDE 

Early radiosonde designers realized that the output of meteorological sensors could modulate the radio signal mechanically 

or electrically. Modulation, in this context, has a broad mean-
32 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 26.—The first "thermoradio" flown by Robert Bureau. From left to right, the windmill and toothed 

wheel for modulation, the half-metal, half-insulator cylinder, the toothed wheel carrying the on-off keying cam 

(i.e., the metal spindle parallel to the cylinder), the arm of the thermometer, and the bimetallic thermometer itself. 

(From R. Bureau, La Meteorologie, volume 7, 1931.) 

ing of changing the radio-frequency carrier signal in some distinctive manner that conveys information. The simplest method 

of modulation is to turn the signal on and off in some timed sequence. A more complex modulation method is to change the 

frequency, phase, or amplitude of the carrier signal. 

In mechanical modulation, displacement of an arm, which 

was the output of standard sensors, such as the bimetallic thermometer or the aneroid barometer, was linked to the transmitter by an electromechanical transducer that altered the radio 

signal. It is easiest for mechanical modulation to switch the signal on and off, but other types of modulation were possible 

and, in fact, Bureau used one such scheme to alter the radiosonde signal frequency with a rotating toothed wheel placed 

close to the transmitter tuning circuit. 

For electrical modulation, special sensors, such as a temperature-sensitive capacitor, were used as components of the transmitter circuit. Changes in the electrical characteristics of such 

components with variation of the atmospheric parameter being 

measured, in turn, altered the radio signal, usually through a 

change in frequency. The earliest application of electrical modulation was the radiosonde flown by Blair in 1924, where the 

frequency of the radio signal was changed by transmitter components that responded to variations in temperature (Blair, 

1937:2). 

Robert Bureau was aware of these design permutations and, 

starting in 1927, he experimented with many of them at his laboratory (Idrac and Bureau, 1927). Bureau flew his first radiosonde, equipped only with a thermometer, on 7 January 1929 

(Figure 26). In the spring of 1929, he flew an instrument (Figure 27) that sequenced the output of a thermometer and a barometer with a 10-toothed wheel (with one tooth missing) 

geared directly to the cylinder on which the thermometer and 

barometer arms operated (Bureau, 1931). The missing tenth 

tooth in the wheel produced a gap in the small frequency modulations signaling a full turn of the cylinder. Therefore, by counting the number of "pips" on the recording produced by the receiver on the ground, the rotational speed was determined. The 

speed was then used to calibrate the on-off "dot" rate produced 

by the rotating cam and cylinder mechanism just as with the 

earlier system but eliminating one of the wheel mechanisms. 

The receiver provided an output current that drove the arm of a 

register that scratched a visual record on smoked-glass (Figure 
NUMBER 53 33 

FIGURE 27.—The "barothermoradio" of Robert Bureau, which was flown in the spring of 1929. The windmill is 

at the top. From left to right are the bimetallic thermometer, the cylinder, and beneath it the cams with their train 

of gears, and a lever, which, under the action of the cams, is supported periodically on the stylus of the barometer, whose capsule is at right. The arm of the thermometer is on the bottom of the cylinder, and the arm of the 

barometer is at the top. (From R. Bureau, La Meteorologie, volume 7, 1931.) 

28). Although Bureau had experimented between 1927 and 

1929 with wavelengths as short as four meters (Figure 29), he 

rejected these higher frequencies because of reception difficulty 

when the line of propagation was near the horizon. Bureau's 

first design was modified over the next decade, but the basic 

principle (the Olland cycle) remained the same, and the longer 

wavelength, 60 meters, was retained. 

It is pertinent to note that Bureau (1931:306) coined the term 

"radiosonde," using it to designate balloon-borne payloads that 

transmit atmospheric parameters (usually pressure, temperature, 

and humidity) to a ground receiver via radio. This is still the 

most common usage of the term, although some writers apply it 

to the balloon and payload as a system. Also, in current usage 

radiosonde can refer to special balloon-borne payloads that 

transmit measurements of nonatmospheric parameters, such as 

cosmic-ray intensity. Those writers who use this narrower definition generally use the term "meteorological radiosonde" to refer to the low-cost units that measure only pressure, temperature, and humidity. In 1938 the U.S. Weather Bureau officially 

adopted the term "radiosonde" to refer to "meteorological radiosondes," in preference to an earlier term "radio-meteorograph" 

(Snyder and Bragaw, 1986:122, footnote 8). 

THE MOLTCHANOFF RADIOSONDE 

At a symposium in October 1931, Soviet scientist Pazel 

Moltchanoff, a scientist at the Institute of Aerology at Sloutzk, 

U.S.S.R., reported the successful flight of a balloon-borne 

transmitter that relayed temperature and pressure data to a 

ground receiver. According to Beelitz (1954:69), Moltchanoff 

also stated that this flight "was the first one in the world where 

a radiosonde was used for exploration of the atmosphere." This 
34 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

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FIGURE 28.—Recording strips produced by the Bureau radiosonde. Strips 1, 2, and 3 are successive parts of the 

same strip and show the transmitted signal as flat, noise-free portions of the trace. From A to B, the flat portions 

occur when the stylus touches a conducting portion and sends an indication of the thermometer. In C, D, and E, 

the flat portions indicate they are from the barometer. Readings are taken from a number of transmitted (flat) 

pulses, not their individual lengths. Strip 2 is a null selection of the cam, sending neither temperature nor barometric data. The temperature reading is 4 pulses, whereas the barometer reading is 3 pulses. (From R. Bureau, La 

Meteorologie, volume 7, 1931.) 

inaccurate claim was accepted by some contemporary commentators, and it has been reiterated by some modern historians 

as well.7 Therefore, it is relevant to comment on the matter of 

priority. 

The first documented radiosonde flight was made in 1924 by 

Blair, but because this flight was not reported in a scientific 

journal until September 1931, it is understandable that neither 

Bureau nor Moltchanoff were aware of Blair's work. However, 

Bureau published the results of his first flights in 1929 in the 

widely read journal, La Meteorologie. In this publication Bureau (1929:1565) averred that he used a balloon-borne 

short-wave radio "to transmit the indications of a barometer 

and a thermometer also attached to the sounding balloon." Apparently, in 1931 Moltchanoff did not know of Bureau's flight. 

The failure, however, of some modern historians to cite the 

prior radiosonde flights of Blair and of Bureau is less understandable. 

Moltchanoff's interest in meteorological applications of radio began in 1923, and in 1928 he published a paper suggesting 

the possibility of transmitting the output of meteorological sensors by radio (Moltchanoff, 1928:39). By 1930, he had constructed a radiosonde (Figures 31, 32) that became known as 

the Kamm-geraet, or "comb-apparatus," because of the appearance of its contact mechanism. The most interesting feature of 

the Moltchanoff sonde was that each of the contact systems 

caused the radio transmitter to relay a specific Morse Code letter by on-off keying to the ground receiver: the first four combs 

were assigned the letters e, i, s, and h. The fifth comb, whose 

purpose was to provide a reference point, was assigned the letter g. The Morse letter picked up by the receiver clearly identified which of the combs was in contact with the temperature 

arm. Because the temperature difference corresponded to the 

distance between two teeth, the rise or fall of temperature was 

determined by listening with an audio-output receiver to the 

Morse codes produced. In practice, the radio operator wrote 

down the sequence of Morse letters and later decoded them 

into numerical values of pressure and temperature. 

The Moltchanoff sonde used essentially the same circuit as 

the Bureau sonde but with a half-wave dipole antenna. The antenna was hung from the balloon, with the sonde itself in the 

middle, like Blair's 1924 tracking transmitter. Moltchanoff first 

flew this radiosonde on 30 January 1930 at Sloutzk (Moltchanoff, 1933). The sonde reached a height of 10 kilometers and 

measured temperature and pressure within this altitude range. 

A feature of the Moltchanoff sonde was that its signals could 

be heard on the conventional audio-output radio receivers, 

therefore no special equipment was needed. This advantage, 

however, was offset somewhat by the electromechanical complexity of the radiosonde that was necessitated by the encoding 

scheme. 

THE DUCKERT RADIOSONDE 

On 22 May 1930, a few months after the first flight of 

Moltchanoff's radiosonde, Paul Duckert, of the Lindenberg 
NUMBER 53 35 

FIGURE 29.—Later model of the Bureau radiosonde incorporating a clockwork-driven cylinder. (From P. Beelitz, 

Radiosonden, Berlin, 1954, fig. 25.) The transmitter on Bureau's sensor-less flight in 1927 had used a wavelength of 42 meters and a quarter-wave antenna. Transmitters on the 1929 flights used a wavelength of approximately 60 meters (4.8 MHz) and retained the quarter-wave antenna design, which allowed the radiosonde to be at 

the balloon, with the antenna extending down away from it. In later models, the cylinder was rotated by a constant-speed, spring-operated clockwork, and provision was made for the inclusion of a hair hygrometer to make 

the instrument capable of measuring humidity as well as temperature and pressure. Bureau's work to simplify 

transmitter circuitry and to find a suitable wavelength-established design—a single triode Hartley oscillator 

directly coupled to a quarter- or half-wave antenna with dry plate and filament batteries—was followed by 

almost all investigators during the next decade. 

Observatory in Germany, flew a radiosonde of his own design. 

The sonde, which attained a height of 15 kilometers, transmitted measurements of pressure and temperature to a groundbased receiver (Duckert, 1932). 

Duckert's radiosonde differed from those of Bureau and of 

Moltchanoff in that it used a thermometer to vary the capacitance of the transmitter inductor-capacitor (LC) tuning circuit. 

This capacitance change varied the frequency of the radio signal. The pressure was measured by means of a Bourdon tube 

and was indicated by interruptions of the signal. Blair had pioneered the method used by Duckert to measure temperature by 

varying the frequency of the signal with a temperature-sensitive 
36 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 30.—A production version of the Bureau radiosonde, circa 1932, marked "Constructeurs Mecaniques et 

Electriques P. de Presale 104-106, Rue Oberkampf, Paris." It is in an aluminum foil covered box, 27x16x13 cm; 

the vacuum tube and batteries are missing. (Instrument in NMAH collection, catalog number MHT 319827; 

Smithsonian photo 61837-D.) 

component of the transmitter circuit. Duckert, however, could 

not have been aware of Blair's unpublished work and independently reintroduced the concept in a more sophisticated form. 

The Duckert sonde used a bimetallic thermometer, which 

varied a capacitor, and a Bourdon tube barometer coupled to 

the transmitter by a simple sliding switch (Figure 34). Duckert's method of coding, direct-frequency modulation of the 

transmitter carrier, was extremely difficult to perfect, consistent with stability and precision, as Blair had discovered several years earlier. It required carefully engineered components 

to eliminate all environmental influences on the transmitter frequency except those calibrated as part of the sensing function. 
NUMBER 53 37 

FIGURE 31.—A Pazel Moltchanoff radiosonde. The method of suspending a complete sonde and antenna is 

shown at the lower left. In this instrument, a series of six star-shaped cams with various numbers of points was 

operated by a windmill through gearing. The choice of a cam, which made contact at a given moment, was made 

by a second switch operated by the appropriate meteorological sensor. The windmill also operated a flat, insulating disk with switch points and a switch arm. This formed part of the humidity circuit and had other functions. 

The instrument box is made of aluminum, measures 36 cm long x8 cm widex 15 cm high, exclusive of the windmill. (From P. Moltchanoff, Gerlands zur Beitrage Geophysik, volume 34, 1931.) 
38 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 32.—Mechanical parts of Moltchanoff's first radiosonde design showing the rotating contact and sensors. (Instrument in NMAH collection, catalog number MHT 316262; Smithsonian photo 48467-A.) 

In a later model (Figure 35), all circuit components (except for 

the capacitor linked to the thermometer) were vacuum sealed in 

a glass tube; a feature that significantly improved performance 

in the hostile operational environment of radiosondes. By the 

mid-1930s, the cumbersome thermometer-capacitor assemblage was replaced by a simple capacitor with a temperature-sensitive dielectric. Provision for measuring and transmitting humidity was included (Duckert, 1932, 1933, 1935). 

The instruments described above constitute three distinct 

types, according to the means for converting the indications of 

the sensors into modulation of the radio transmissions. The 

three methods are chronometric (Bureau), coding (Moltchanoff), and frequency modulation (Duckert). From 1931 to 

1940, more than one dozen radiosonde designs were manufactured (see Beelitz, 1954), but all were based upon one of these 

three methods. These three designs can be considered prototypical of the instruments that followed. 

These categories are not fundamental in a radio engineering 

sense because they do not represent mutually exclusive ways in 

which to code and modulate information onto a radio transmis-
NUMBER 53 39 

FIGURE 33.—Paul Duckert's first radiosonde. A continuous radio signal was emitted whose frequency varied 

with temperature, whereas pressure was indicated by interrupting the signal by a Bourdon tube (c) and a toothed 

wheel (d). One plate of the capacitor (b) was moved by a bimetallic thermometer (a and h). The complete radio 

transmitter was inside the glass tube (f). (From Duckert, Beitrdge zur Physik der Freien Atmosphare, volume 18, 

1932.) 

sion. Nevertheless, they represent the basic categories into 

which radiosonde evolution can be divided during the period 

from 1931 to 1940. The basic radio-frequency design of numerous radiosondes, both during the prototype phase and during later development, underwent remarkably little change. 

The transmitters, with rare exceptions, used a single triode connected as a radio-frequency oscillator with the inductance-capacitance tank circuit coupled to a resonant antenna. 
40 

FIGURE 34 (right).—Diagrams showing the method of measuring temperature 

and pressure used in Paul Duckert's first radiosonde. In the upper diagram a 

bimetallic thermometer (T) by means of mechanical linkage (A) varied the 

capacity of the capacitor (C) in the oscillating circuit, which in turn varied the 

emitted radio frequency. In the lower diagram, the Bourdon tube (T) expanded 

and caused the contact arm (A) to pivot about its center and slide along segment (S) touching the contacts (C) intermittently. Dashes were therefore transmitted while the arm touched the contacts corresponding to steps of pressure 

change. (From Wenstrom, Monthly Weather Review, volume 62, 1934). 

FIGURE 35 (below).—A Paul Duckert radiosonde, circa 1932. The entire radio 

circuit, except for the variable capacitor, operated by a bimetallic thermometer, 

is enclosed in a vacuum tube 6 cm in diameter and 16 cm long. This 

"brute-force" approach to isolating the transmitter from temperature and 

humidity changes can cause spurious frequency changes. A gold-plated, 

toothed wheel, turned by a Bourdon tube, switches between the variable capacitor and a fixed capacitor at known values of pressure. Overall dimensions are 

16x8.5x31 cm. (Instrument in NMAH collection, catalog number MHT 

313527; Smithsonian photo 48467-C.) 

SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

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6. Maturity, 1931-1940 

The importance of radiotelemetry as a meteorological tool 

was widely recognized after Bureau, Moltchanoff, and Duckert 

demonstrated the various ways that the output of conventional 

balloon-borne sensors could be transmitted by radio. Thereafter, further radiosonde development mainly involved the development of meteorological instruments that were capable of 

providing synoptic atmospheric data at low cost, although research included exploration of the possible geophysical applications (see below). By the 1940s, radiosondes could be called 

"mature" in so far as being well-engineered devices suitable for 

mass production. 

CHRONOMETRIC RADIOSONDES 

In 1931, Moltchanoff, whose earlier sonde was a coding 

type, designed a radiosonde that was based upon the Olland 

chronometric system, which was manufactured by the Askania-Werke in Germany (Heck and Sudeck, 1931). In this sonde 

(Figures 36-38), the sensor-sequencing contacts were actuated 

by a clock, and the means of synchronization were incorporated so that the radio signals emitted by the sonde could be received by a facsimile recorder. 

This was the first radiosonde to provide such recordings for 

all three parameters of meteorological interest. The sonde, 

however, was heavy, complex, and relatively expensive. Moreover, the accuracy of the data depended upon the constancy of 

the clock that drove the rotating contact and thus determined 

scanning speed, a source of imprecision in all sondes based 

upon the Olland System. 

FIGURE 36 (right).—A Moltchanoff-Askania radiosonde. Shown in outline are 

the transmitter (T) and its small batteries (B) for filament heating and anode 

supply. The frequency of the transmitter was controlled by a quartz crystal to 

maintain the constancy of frequency over the range of temperatures encountered, an unusual design among early radiosondes. The three measuring elements are the barometric diaphragm (A), the thermometer (M), and the 

hygrometer hairs (C). The thermometer and the hygrometer are housed in the 

air channel (G), whereas the former is protected from radiation by a highly polished cylinder (H). A clockwork motor drove the contact arm (D); holes were 

for winding and control in the cover (F). These are all supported on a plate (E) 

in the upper part of the casing. The transmitter and its batteries sit on an insulating sheet on the bottom-plate. The plug (K) completed the circuit to the contact arm and also to the leads (L) of the dipole antenna. Note the mechanical 

arrangement for suspending the sonde at the center of the antenna, an awkward 

design that contributed to excessive swinging of the sonde below the balloon 

during ascent. (From J.F.H., Nature, volume 130, 1932.) The radiosonde 

weighed two kilograms and employed a short-wave transmitter that operated in 

the range between 25 and 100 meters. The sonde was positioned at the middle 

of a half-wave dipole antenna that trailed below the balloon. Three sensors 

were used: a bimetallic thermometer, a Bourdon-tube barometer, and a hair 

hygrometer. 

In 1935, Bureau produced a design that eliminated the undesirable effects of small variations in scanning speed by changing from a windmill drive of the scanning disc to a small electric motor and by combining this with his earlier method for 

determining the rate of rotation of the scanning disc (Anonymous, 1961:100). A frequency modulation was superimposed 

41 
42 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

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FIGURE 37.—Control mechanism in the Moltchanoff-Askania sonde. The contact arm (A) was rotated 

continuously by a clockwork drive. Three pointers of appropriate measuring elements were arranged 

one above the other so that they moved on a uniform circle, with the top of each pointer making a 

fleeting contact with the brush (A) in the course of its rotation. The brush also made contact on the 

synchronizing segment (S) and on fixed contacts (CI and C2), which jointly marked its revolution into 

three parts labelled temperature, barometer, and humidity, respectively. The relatively long contact at 

S was for the purpose of sending a longer signal in order to synchronize the recording cylinder at the 

receiver in a manner well known in connection with facsimile receivers of the period. The time interval between the end of the synchronizing signal and contact with the temperature arm thus depended 

upon the instantaneous position of the latter; similarly, the interval between the signal from CI and 

that from the barometer arm depended upon the positions of this pointer. This control was based upon 

the Olland System. The transmitter was an on-off keyed, CW type, where the keying contact was 

sequentially coupled to various sensors. 

on the radio carrier from timing pulses of sufficient rate (about 

50 per second) to permit the speed of rotation of the scanning 

disc to be measured by the number of pulses. The frequency 

modulation was produced by a toothed wheel near the transmitter tuning circuit turned by the scanning disc. During the time 

the movable and fixed arms contacted the scanning disc, the 

pulses were suppressed and the angular motion of the disc was 

measurable, not as a function of time, but in terms of the number of pulses between contacts. A tape-chronograph recorder 

was used to record the signals. Pulse counting was facilitated 

by the fact that every tenth pulse was suppressed; an effect produced by the omission of one tooth in 10 on the toothed wheel. 

In Germany, A. Lang developed a radiosonde that utilized 

the chronometric method of transmitting measurements of temperature and humidity by interrupting the radio signal (Weickmann, 1937:28). The Lang sonde (Figures 39, 40), which appeared in 1936, represented an attempt to reduce the size and 

weight of the transmitter and the antenna. The ground receiver, 

as in the case of the Moltchanoff-Askania sonde, was coupled 

to a facsimile machine to produce a visual record of the data. 

These clockwork-regulated chronometric radiosondes had a 

common problem—the discontinuous motion of the clock limited the precision of any reading. In 1939, R.C. Jacobsen introduced a constant-speed motor, which greatly increased the possible number of discrete movements in a radiosonde (Figures 

41, 42) used by the Canadian Weather Service (Middleton and 

Spilhaus, 1953:246). Both the Bureau and the Jacobsen mechanisms represented mature chronometric design and were manufactured in quantity in the following decades, as was the Lang 

radiosonde for the German Meteorological Service. 
NUMBER 53 43 

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FIGURE 38.—Facsimile of recordings from a Moltchanoff-Askania radiosonde. Scale was applied by reference to the beginning of signals from fixed contacts, designated CI and C2 in Figure 37. On the ground, 

signals were picked up by a conventional receiver coupled to a facsimile recorder of the rotating-cylinder 

type. The facsimile machine produced a visual record of transmitted data produced by the three sensors in 

the sonde and displayed this data in a manner that allowed the records to be turned into Cartesian graphs 

by the addition of coordinate scales. 

CODING RADIOSONDES 

The first Moltchanoff coding radiosonde, with minor improvements, was used extensively in the U.S.S.R. for several 

decades (Anonymous, 1961:100). Although a few articles reported schemes for translating radiosonde sensor outputs into 

telegraphic code, the complexity and expense of including coding in these radiosondes precluded their adoption elsewhere. 

Their advantage was that the coded signals could be picked up 

by any conventional receiver with an audio output. 

The only new coding sonde developed during the 1930s was 

produced by J. Gaw, a German engineer. In this sonde, one of 

several types used until the 1940s by the German Meteorological Service, a small electric motor rotated an insulated cylinder, 
44 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 39.—Diagram of the mechanical parts of an A. Lang radiosonde and 

fragmentary transmitter circuit. The reference contact (Z) is adjacent to the 

hygrometer arm. An aneroid capsule and pressure arm switch are shown at the 

top. A starred-wheel temperature contact and rotating cylinder (W) are at the 

bottom of the figure. (From L. Weickmann, Denkschrift Uber Radiosonde-Konstructionen, Berlin, 1938.) The sensors were a bimetallic thermometer, an aneroid barometer, and a hair hygrometer. The novel feature was that, 

instead of a single contact arm, the bimetallic thermometer rotated a starshaped contact; an arrangement that, in effect, expanded the temperature 

scale. The hair hygrometer moved an arm that, in conjunction with a fixed 

contact, shut down the transmitter once each revolution for a time period 

indicative of the humidity. The clock drove two coaxial cylinders, one with a 

conducting band parallel to the axis, the other a conducting helix. Both temperature and humidity were based upon a time-cycle principle, but the aneroid 

barometer simply operated a switch that interrupted the radio signal at predetermined pressure values, thus allowing the sequence of interruptions to be 

interpreted in terms of pressure. (Smithsonian photo 63882.) 
NUMBER 53 45 

FIGURE 40.—An A. Lang radiosonde, with a short-wave transmitter (10.81 meters; 27.75 MHz), which weighed, 

with the mechanical arrangements described in the text, 560 grams. The wavelength used was considered very 

short in the 1930s. The mechanical arrangements are in the upright casing; the radio transmitter is in the cylinder. 

(Instrument in NMAH collection, catalog number MHT 322211; Smithsonian photo 61836.) 

portions of which had a conducting surface. Contact arms from 

a bimetallic thermometer, aneroid barometer, and hair hygrometer touched conducting portions of the rotating cylinder in a 

way that produced combinations of two Morse letters for each 

sensor. The complete cycle for the three sensors was repeated 

eight times per minute. Signals could be heard on the ground 

by using an ordinary radio receiver and could be written down 

as Morse letters that were interpreted later as values of pressure, temperature, and humidity. 

FREQUENCY MODULATION RADIOSONDES 

The Duckert radiosonde underwent various modifications 

during the 1930s (Duckert, 1932, 1933, 1935:573-584), including replacement of the oil-filled variable condenser used as 

a temperature sensor by an "electric" thermometer consisting 

of a temperature-sensitive capacitor. This was made possible 

by the development of a material whose dielectric constant had 

a large temperature-coefficient. Such material then was used as 

the nonconducting portion of a capacitor whose capacitance 

became a predictable function of temperature. Other Duckert 

sondes, unlike the first model that only measured temperature 

and pressure, incorporated a hygrometer and a means for transmitting humidity by varying the length of time the radio frequency was turned on. In later models, the pressure switch, 

which in earlier designs simply interrupted the signal, switched 

from the variable capacitor to a fixed-reference capacitor in order to provide a means of monitoring transmitter stability. 

These later designs also included ceramic insulation to make 

the transmitter more stable. The re-engineered designs of the 

Duckert sonde were mass produced throughout the 1930s by 

the German firm of Telefunken. 

Radio-frequency modulation, which Blair had used in his radiosonde of 1924, and which Bureau mentioned in an article 

published early in 1931, was used in Duckert radiosondes as 

early as 1930. That same year Vilho Vaisala, a Finnish engineer, 

independently conceived of frequency modulation as a simpler 

alternative to the Moltchanoff coding sonde (Vaisala, 1932). 
46 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 41.—R.C. Jacobsen's radiosonde, designed for the Canadian Weather Service in 1939. The pressure and 

temperature arms were provided with special quick-break devices (K.) shown in inset. The wire was attached to 

the arm by insulating cement and completed the circuit through a spiral only for the moment after contact, but 

prior to the point where the lateral pressure pushed it away from the arm. (From W. Middleton and A. Spilhaus, 

Meteorological Instruments, 1953, fig. 171.) The Jacobsen sonde employed a spiral contact and an improved 

electric motor whose rotational speed was stabilized by a weighted reed. The rotating contact (A) was driven at 

about four revolutions per minute by the electric motor. A magnet (B) rotated inside a rectangular coil (C). The 

eccentric on the shaft of the magnet (B) vibrated a spring against an adjustable contact. Because the natural frequency of the spring was 1500 vibrations per minute, the shaft of the motor rotated at 1500 revolutions per 

minute with great constancy, thus reducing this source of imprecision. The frame of the sonde was insulated from 

the rotating spiral contact and supported an aneroid barometer (E), a bimetallic thermometer (F), and the goldbeater's skin hygrometer (G). 

A major problem in using frequency modulation was the 

variations, unrelated to the sensors, in oscillating frequency. 

Such an instrument required a highly stable transmitter free 

from frequency changes, such as those produced by variations 

in battery voltage or transmitter components. The Duckert 

sonde had achieved the requisite stability, but Vaisala's solution 

to the problem was more innovative. His design (Figures 43, 

44) provided standard reference frequencies in the transmitted 

signal to alleviate errors caused by spurious frequency changes 

in the oscillator circuit (Vaisala, 1932:10). The Vaisala sonde 

(Vaisala, 1935) was light (365 grams, including battery) and inexpensive. It was used by the Finnish Weather Service in large 

numbers beginning in the mid-1930s and was used with only 

minor design improvements during the ensuing years. 

In 1937-1938, the British National Physical Laboratory 

(NPL), under the leadership of L.A. Thomas, perceiving the 

need for both direction-finding equipment and electrical sensors,8

 experimented with audio frequency. The sensors consisted of two iron-cored coils whose inductance was varied by 

an evacuated, metal, bellows-type barometer and a resistancewire thermometer. 

FIGURE 42.—Diagram of the spiral contact in a Jacobsen radiosonde. T, thermometer arm; P, pressure arm; U, humidity arm; H and J, reference contacts. 

The contact arms of each sensor moved over a spiral so that once in each revolution the end of the spiral touched reference points H and J. In the sonde, the 

stabilized motor reduced inaccuracy attributable to variations in scanning 

speed, and the absence of pivots in the sensor couplings reduced mechanical 

slippage, which was another source of error in sondes of this type. 

This radiosonde had several undesirable features. Because 

each of the three sensors required a separate oscillator, it was 

heavy, a defect magnified by the fact that the method for "read-
NUMBER 53 47 

FIGURE 43.—A Vilho Vaisala radiosonde. Five capacitances were successively switched into the transmitter circuit by a rotating switch driven by a windmill. Overall dimensions of the sonde, exclusive of the windmill, are 

9x 11 x22 cm. In Vaisala radiosondes, a rotating switch driven by a windmill sequentially connected five capacitors to the oscillator circuit of single-tube transmitter. Two of the capacitances were of fixed type and were used 

as references, and the remaining three were variable in accord with sensor output (see Figure 44). Sensors were 

arranged so that as pressure and temperature decreased and humidity increased, the associated capacitances all 

increased and, therefore, radio-frequency decreased. The rate of frequency modulation was quite low relative to 

the carrier frequency, so the bandwidth occupied by these transmissions was essentially the spread between the 

lowest and highest carrier frequencies produced by the switched capacitors, amounting to 1.6 MHz. The carrier 

frequency itself was between 23 and 27 MHz. During each cycle of the switch, two reference capacitors were 

connected to an oscillator circuit as a means of eliminating errors caused by frequency drifts. The mathematical 

basis for making the signals independent of transmitter drift was worked out by Vaisala and can be found in more 

recent texts, e.g., Middleton and Spilhaus (1953:253). In essence, if wavelengths of emitted signals are measured 

on a receiver in which plates of the tuning capacitor are semicircular, so that the square of the wavelength is proportional to the angular reading of the receiver's control knob, then it is possible to obtain the measurement produced by a given sensor in terms of the ratio of differences of successive angular readings of the tuning knob of 

the receiver. In practice, this method substantially lessened errors caused by transmitter drift. Implementation of 

this technique was facilitated by using a semi-automatic recorder which, when triggered manually, registered the 

readings on a clockwork-driven drum. (Instrument in NMAH collection, catalog number MHT 319825; Smithsonian photo 61837-C.) 

ing" sensor output required the variation of the inductance in 

the LC circuit of the oscillator, which worsened the weight 

problem. The NPL radiosonde also was expensive to manufacture and was never produced in large quantities. Instead, in 

1940 a new program, begun at Kew Observatory by E.G. Dymond and his associates, developed the Kew radiosonde (Figure 46), a successful design that was mass produced.9 

It is important to note, however, that the NPL design, although not effective for meteorology, led directly to the development of a whole new field of technology—multichannel data 

telemetry. (The role of the NPL sonde in this development is 

discussed in Section 7.) 

During the 1930s, the meteorological radiosonde advanced 

from the prototype stage to mature designs suitable for mass 

production. There was a marked tendency in France, the Soviet 

Union, and Finland to improve on the prototypes developed in 

each country, based respectively upon chronometric, coding, 

and frequency-modulation principles, rather than to experiment 

with other possibilities. This tendency was less pronounced in 

Germany, where sondes in all three categories were used: the 
48 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

ay - 5ov.|+ 5ov. 

FIGURE 44.—Schematic diagram of a Vaisala radiosonde: 1, barometer housing; 2, thermometer housing; 3, hair 

hygrometer; 4, variable capacitor of aneroid barometer; 5, variable capacitor of bimetallic thermometer; 6, variable capacitor of hygrometer; 7, 8, reference capacitors, 9, rotary switch. The oscillator, which was frequency 

modulated by sensors, was a common triode inductive plate-grid feedback type. Note the symmetrical, half-wave 

antenna required the sonde to be centered between each half of the antenna. (From L. Weickmann, Denkschrift 

Ober Radiosonden-Konstructionen, Berlin, 1938. Smithsonian photo 63881.) 

Askania-Werke and the Lang (chronometric), Duckert (carrierfrequency modulation), and Gaw (coding). Even so, however, 

the Duckert sonde predominated in Germany and was successively re-engineered over the years. 

With regard to cost, the adoption and deployment of any type 

of radiosonde required training personnel in its use and establishing extensive ground installations. Although the sondes 

were relatively cheap, the cost of training, of organizational infrastructure, and of ground equipment represented a substantial 

investment that would be largely lost by a radical change in the 

sonde. It is significant that in Germany, where different types 

of sondes were used, a massive military rearmament program 

was underway during the 1930s.10 Thus, the more ample funding available for satisfying military meteorological needs in 

Germany may account for the more eclectic approach adopted 

in that country. In the United States, the chronometric types 

were developed first, but before any large-scale use was made 

of them the NBS-Navy sonde (see below) appeared and became the unit of choice. Thus, in the United States the 

changeover to a radically new design was made before any 

substantial investment in training and ground equipment had 

been made. 

Another factor that influenced the retention of the original 

designs was that by 1940 the representative sondes in each category were "good enough."11

 This fact, plus the ancillary costs 
NUMBER 53 49 

•-® 

SCALE 

FIGURE 45.—A late model NPL radiosonde. 1, support bracket; 2, variable 

inductance; 3, thermometer shield; 4, batteries. This radiosonde was among 

the earliest ancestors of the multiple audio-subcarrier type that led to modem 

analog data telemetry. (From H. Thomas, Proceedings of the Royal Society, 

London, volume A167, 1938.) The design of the Thomas-NPL instrument, 

favored by the Meteorological Office at Kew, was based upon particular considerations: it used less of an already overcrowded radio-frequency spectrum 

when compared with direct-carrier frequency modulation and was less 

affected by interference. Nevertheless, although L.A.Thomas adopted the 

basic system pioneered by H. Diamond, he rejected the use of the barometric 

switch employed in the NBS sonde. (Apparently, Thomas had tested Duckert's sonde that used barometric switching and found it unreliable.) Instead, 

Thomas opted to amplitude modulate the signal simultaneously with two discrete audio frequencies representing the thermometer and barometer outputs. 

Separate "beat frequency" oscillators in a superheterodyne ground receiver 

could be alternately connected and subsequently adjusted by an observer to 

reach a "zero-beat" condition, thus allowing temperature or pressure data to 

be obtained. In this model, a third coil and oscillator, whose inductance was 

varied by a goldbeater's skin hygrometer, was added. Use of variable inductances as sensors simplified the design of the audio-subcarrier oscillator but 

raised problems of weight and cost. 
50 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 46.—British Meteorological Office radiosonde, Kew MK2. The transmitter and batteries are in a black, 

molded-fiber box, 12 cm in diameter and 23 cm high. The sensors are contained in rectangular aluminum shields 

mounted on three sides of the box. On the fourth side is a wheel with three conical cups, which revolved in the 

airstream to operate a switch that connected the three meteorological sensors in sequence. The Kew radiosonde, 

however, unlike its NPL predecessor, did not continuously transmit signals associated with the three sensors. 

Instead, the sensors were sequentially switched into the transmitter circuit by means of a rotating contact driven 

by a windmill. This latter feature was similar to the switching arrangement used in the Vaisala sonde. The fact 

that signals associated with each sensor were transmitted sequentially meant that only one audio-frequency oscillator was required. This made the sonde lighter and less costly than the NPL model. Also, two of the sensors used 

in the NPL sonde were redesigned for the Kew model—an aneroid barometer was substituted for the bellows-type, and a resistance-wire thermometer was replaced by a bimetallic type. Until the mid-1930s, the various 

radiosondes used in the United States were essentially elaborations of the chronometric design. In 1936, however, a radiosonde was developed in the United States that introduced frequency modulation of the audio-frequency subcarrier. In this method, the amplitude of an audio-frequency-tone modulated the radio carrier signal. 

The tone was then frequency modulated. (Instrument in NMAH collection, catalog number MHT 322313; 

Smithsonian photo 61906-B.) 

in training and equipment, provide a plausible rationale for the 

course of development from 1931 to 1940 and later. 

AMERICAN RADIOSONDES 

The radiosonde instruments in the collection cataloged 

herein came from American institutions, principally from the 

United States Weather Bureau. They consist, not surprisingly, 

of more-or-less final versions of foreign instruments, plus 

American instruments, most of which never were made in 

quantity and were considered experimental. In order to make 

this narrative more comprehensible, we have separated these 

two categories. Our general history of radiosonde development 

has been based largely upon "mature" foreign instruments, 
NUMBER 53 51 

FIGURE 47.—A Blue Hill radiosonde, circa 1935 (from Lange, Bulletin of the American Meteorological Society. 

volume 17, 1936). As the rotating cylinder, clearly shown in the center, revolved, the wire of the sensor arms 

sequentially contacted the arms of an aneroid barometer, a bimetallic thermometer, and a hair hygrometer. This 

arrangement resembled that used by Herath and Robitzch in 1917 and by Bureau in 1929. 

leaving for separate discussion the American instruments, 

about which it is possible to give more detail on experimentation with different designs. 

In the United States, the Blue Hill Observatory, near Boston, 

Massachusetts, produced in 1935 a chronometric radiosonde 

(Figures 47, 48) that utilized an insulated, clock-driven cylinder with an inset helical wire as the rotating control contact. 

The next year similar instruments were produced by the 

Guggenheim Aeronautics Laboratory of the California Institute 

of Technology (called GALCIT) (Maier and Wood, 1937) and 

by Friez, an established maker of meteorological instruments in 

Baltimore, Maryland. Both mechanisms were similar in design 

to the early Bureau sondes. 

The designers of the Blue Hill and GALCIT sondes solved 

the chronological problem by using special clocks with a relatively large number of discrete movements per revolution. The 

U.S. Bureau of Standards (NBS) replaced the clocks with 

small, constant-speed electric motors, including Jacobsen's 

constant-speed motor (Curtis and Astin, 1935, 1936:358). In 

1937, a radiosonde featuring audio-frequency subcarrier modulation was developed at the NBS (Diamond et al., 1938, 1940). 

The Americans finally did develop a mature radiosonde. In 

1936, the U.S. Navy asked NBS to develop a radiosonde that 

FIGURE 48.—A late model of the Blue Hill radiosonde, type F, circa 1936. The 

small bimetal thermometer, hair hygrometer, and aneroid barometer moved the 

contact arms along a cylinder, around which were wrapped three turns of a 

helical conducting wire. The shaft carrying this helix protruded through the top 

of a light wooden box, which was intended to contain a clock, a radio transmitter, and batteries. The box measures 18x10x7 cm. (From Lange, Bulletin of 

the American Meteorological Society, volume 18, 1937; see Figure 47; instrument in NMAH collection, catalog number MHT 322324.) 
52 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 49.—Complete receiving and recording equipment for the NBS-Navy radiosonde as it appeared in 

December 1936. The electronic frequency meter at the center indicated the audio frequencies controlled by the 

effective values of air pressure, temperature, and humidity, thus revealing the upper-air conditions. (From W. 

Snyder and Bragaw, Achievement in Radio, 1986.) Coupled with the development of the radiosonde and new 

sensing elements, the NBS group designed and constructed special ground-station equipment. The complete 

assembly consisted of a superregenerative receiver coupled to an electronic audio-frequency meter through an 

amplifier and electrical filter unit. The output of the receiver was applied to the input of a high-speed recording 

voltmeter. In effect, the assembly converted audio-frequencies into a graphical chart whose abscissa could be calibrated directly in terms of the measured parameters. 

would meet the Navy's meteorological requirements. H. Diamond and his associates at NBS organized a well-financed program that produced a superior radiosonde (Diamond et al., 

1937). The thermometer used in the first NBS sondes was a 

small glass tube filled with a nonfreezing electrolyte. This electrolytic temperature sensor could be calibrated to a few tenths 

of a degree Centigrade and was stable across the range of environmental conditions encountered by the radiosonde. The mechanical sensors used to measure temperature and humidity 

were inadequate at the higher altitudes attained by these balloons in 1936. This prompted the NBS to develop of electric 

sensors for both parameters, and the availability of these sensors influenced the design of the NBS-Navy sonde.12 

Although the thermistor temperature sensor was available by 

the late 1930s, when radiosonde production began, it was not 

sufficiently precise and repeatable to be used at that time. By 

the early 1950s, however, thermistor development had advanced, and glass-encapsulated types, with long-term stability 

and precision to less than one degree Centigrade variance, offered sufficient advantage to be used in radiosonde designs. 

The first ground receiver was available in early 1938, and 

service use of the first NBS-U.S. Navy radiosondes (Figure 49) 

began in the Department of the Navy in June 1938, at the Anacostia Naval Air Station in Washington, D.C. (Snyder and 

Bragaw, 1986:127-129). In 1939-1940, the previously described modifications were made to accommodate the electric 

hygrometer. By 1940, the design had been refined, and in that 

year 15,000 such sondes were manufactured (Snyder and 

Bragaw, 1986:127-129). Many hundreds of thousands were 

produced in the following years. 

An important feature of the sonde was a so-called "pressure 

drive." Diamond and his colleagues had sought to avoid the 

complexity and cost entailed by the use of clockwork or electric motors. Some means of switching was required, however, 

and because they had decided to retain the aneroid barometer 

as the pressure sensor, they developed a method using its mechanical displacement as a switch, as well as an indication of 

pressure (Snyder and Bragaw, 1986:127-129). Moreover, meteorologists favored a record in which temperature and humidity were given as functions of pressure. By using the barometer 

as a switch (Figure 50), such a record could be obtained. To 

transmit data, a 65-megahertz (MHz) radio signal was amplitude-modulated at an audio frequency of a few hundred cycles 

per second by a resistance-capacitance (RC) controlled oscillator. Resistive thermometer and hygrometer elements produced 
NUMBER 53 53 

FIGURE 50.—An electric hygrometer and thermometer assembly used in later NBS-Navy radiosondes, circa 

1939. The thermometer is suspended between two arms at the top of the unit; the hygrometer is the upright strip 

in the center. The hair hygrometer was replaced in later models by a newly developed electric hygrometer. This 

consisted of a polystyrene strip coated with a film of lithium chloride dissolved in polyvinyl acetate with two tin 

electrodes sputtered along the long edges of the strip. A combined electrical hygrometer and thermometer were 

mounted on a single assembly that could be easily attached to the sonde. Replacement of the hair hygrometer 

with an electric type necessitated circuit changes that involved the use of a relay and a twin-triode vacuum tube, 

so the final circuit became more complex than that shown in Figure 49, but its essential operation remained the 

same. (Instrument in NMAH collection, catalog numbers MHT 327975 and 327976; Smithsonian photo 61553.) 

a calibrated modulation of this audio-frequency in response to 

temperature or humidity. 

This design was remarkably successful because the meteorological radiosondes used today in the United States still reflect 

the basic design of the NBS-Navy sonde developed more than 

half a century ago. The success of the NBS design can be attributed in part to the Navy's insistence on focusing the development on three factors: cost, weight, and accuracy. The sonde 

was to cost about $25 when purchased in reasonable quantity, 

and the weight was not to exceed one kilogram. It was to measure temperature to within one degree over the range from plus 

40 to minus 75 degrees centigrade, pressure to within one millibar from 1050 to 1500 millibars, and humidity to within three 

percent from 0 to 100 percent relative humidity. In addition, the 

U.S. Navy imposed a requirement that significantly affected 

not only transmitter design, but also the method of signal transmission. The radiosonde was required to emit signals that could 

be used, in conjunction with radio direction-finding equipment 

on the ground, to determine wind speed and direction (Snyder 

and Bragaw, 1986:131). 

This last requirement meant that it would be advantageous to 

have a continuous signal on a constant carrier frequency to fa-
54 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 51.—This NBS-Navy radiosonde was manufactured in large quantities by 1940 and incorporated 

improvements such as an electric hygrometer. The batteries, transmitter, and pressure unit were housed in an 

insulated box to minimize temperature changes. The temperature tube and electric hygrometer (at rear) were 

shielded against solar radiation by a double-walled metal tube. The total weight was less than 1 kg; the cost was 

$25 in 1940. The finder of the balloon after its flight received a monetary award upon return of the radiosonde. 

(From Snyder and Bragaw, Achievement in Radio, 1986, fig. 52.) 

cilitate direction-finding. It also required, indirectly, higher 

transmitter frequencies that would allow smaller and more accurate tracking equipment. The necessity to transmit information from the sensors on a constant and stable frequency suggested the use of an audio-frequency subcarrier modulated onto 

the main radio-frequency carrier. The subcarrier audio frequency then would be modulated as a means of relaying pressure, temperature, and humidity data. The use of audio-frequency modulation, in turn, made it possible to use circuits that 

were especially compatible with the use of electrical sensors 

based upon resistance that had become available recently. 

These were superior to the mechanical sensors that previously 

had been used in most radiosondes. 

One commentator (Middleton, 1969a:339) suggested that the 

decision of the NBS group to base their sonde on audio-frequency modulation was influenced by the commercial develop-
NUMBER 53 55 

FIGURE 52.—A "pressure drive" of an early NBS-Navy radiosonde. Overall dimensions are 13x9x6 cm. In 

order to understand the operation of a pressure drive it is helpful to view a schematic (see Figure 53) of one of the 

first models of an NBS sonde. This early model employed an electric thermometer, but the electric hygrometer 

was still under development so a conventional hair hygrometer that mechanically varied a resistor (see Figure 

54) was used instead. In this arrangement the arm of the barometer (P) swept over a series of conducting strips 

numbered 1,6, 11, etc., in Figure 53. These strips were mounted on a nonconducting board. The fixed resistor 

(R) and variable resistors (H and T), representing the hygrometer and the thermometer, respectively, were connected in series between the grid of a multi-electrode vacuum tube and the instrument frame. When the barometer arm was not touching a contact, the only path from the grid to the frame was through R, H, and T, in series. 

Thus, total resistance varied with that of the temperature-dependent resistor (T) and so the modulating audio-frequency signal indicated temperature. When the barometer arm touched the unnumbered contacts, the resistance 

was equal to R plus that of the humidity-dependent resistor (H), and audio-frequency was indicative of relative 

humidity. When the arm touched contacts 11 and 26, the point (a) was connected directly to the frame, causing 

the modulation to assume a fixed upper frequency; when it touched 1,6, 16, and 21, the point (b) was connected 

to the frame, producing a fixed lower frequency. Alternate change-overs from one set of frequencies to another 

indicated that the arm was just reaching or just leaving one of the intermediate contacts and had attained deflection position corresponding to a known pressure. (Instrument in NMAH collection, catalog number MHT 

312800; Smithsonian photo 61836-B.) 

ment of a high-quality, audio-frequency meter manufactured by 

the General Radio Company. Of course, advances in radio 

technology in the 1930s influenced radiosonde development in 

all countries. All radiosonde designers responded to the availability of small, light-weight, rugged, vacuum tubes and of ancillary circuit components by repeatedly re-engineering their 

sondes to include such elements. 

Finally, it is pertinent to comment on cost-reduction as a primary consideration in meteorological radiosonde development. 

The early prototypes had demonstrated that radiosondes were 

technically feasible, but their wide-scale deployment as part of 

a synoptic data-gathering program was dependent largely upon 

dramatically lowering unit cost. In the United States, the 

Navy's cost requirement of $25 was relatively low for that 

time, and it was achieved chiefly because of features that made 

the NBS design consonant with the techniques of mass production. This was equally true of the successful sondes that 

evolved within other design categories because procurement on 

a large scale was the most effective means of lowering unit 

cost. 

THE RADIOSONDE IN GEOPHYSICS 

The success of weather radiosondes made scientists increasingly aware of the possibility of transmitting data from a variety of balloon-borne sensors that measured parameters of geo-
56 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

R 

r—*WVWW 

P 

a 

it 

H 

FIGURE 53.—Fragment of a circuit diagram of an early NBS-Navy radiosonde showing the sensor-sequence 

switching mechanism of the audio-subcarrier oscillator. (Smithsonian photo 63263.) 

physical interest, such as ozone concentration or cosmic-ray 

intensity. Therefore, in the decade 1931-1940, further exploration of the upper atmosphere, made possible by the advent in 

1931 of the pressurized gondola for manned balloons, produced information that led meteorologists to place more emphasis on obtaining atmospheric data from very high altitudes. 

The possibility of routinely reaching such altitudes with radiosondes became a reality in 1936 when the first truly low-cost 

latex balloons, capable of lofting radiosondes to great heights, 

became available. These hydrogen-filled balloons weighed 

about 700 grams and were capable of lifting a 1.5 kilogram 

payload to altitudes of about 25 kilometers. 

Although cost was perhaps the most important determinant 

in the design of meteorological radiosondes, it was of little importance in the development of research radiosondes. From the 

mid-1930s to 1940, a number of such research radiosondes 

were flown. A few of these sondes represented original designs, but the others were meteorological radiosondes that were 

adapted to measure a specific parameter of geophysical interest 

(see "Research Radiosondes," below). 

The sequential appearance of each of these special sondes 

did not follow the cost-dictated pattern of design change apparent in the development of meteorological radiosondes because 

each geophysical radiosonde was a one-of-a-kind instrument 

designed to achieve objectives that were, to some extent, 

unique. Nevertheless, these research sondes explored the range 

of application of this type of instrument and, in this sense, their 

development represented an aspect of the maturation of the radiosonde as a measurement tool. By 1940, most of the possible 

applications of geophysical research involving radiosondes had 

been tested. Specifically, sondes for measuring cosmic rays, solar radiation, ultraviolet radiation, and ozone were designed 

and flown successfully. 

The measurement of cosmic rays was one of the first research applications of geophysical radiosondes. The altitude 

dependency of cosmic-ray intensity at various geomagnetic latitudes was of key importance to theoreticians seeking to understand the nature and origin of cosmic rays. Balloonsondes containing self-registering electrometers for measuring cosmic-ray 

intensity had been flown in the 1920s in the United States by 

Robert Millikan (described in Section 1). In the early 1930s, 
NUMBER 53 57 

FIGURE 54.—A hair hygrometer used in an early NBS-Navy radiosonde. Overall length, 20 cm. A cylindrical 

cage of hairs is mounted on the upper surface of light-weight, white plastic molding; on the lower side of the 

molding is a wound resistor, over which a contact was moved by a change in the length of the hairs. (Instrument 

in NMAH collection, catalog number MHT 312799; Smithsonian photo 61901-D.) 

such balloonsondes were flown again by Millikan and also by 

Erich Regener in Germany (Millikan and Bowen, 1926; Regener, 1932). As previously mentioned, however, retrieval of data 

from balloonsondes was always problematical even in populated areas. Moreover, it was necessary to make vertical soundings of cosmic-ray intensity in locales, such as the polar regions, where recovery of a balloonsonde was extremely 

unlikely. 

Radiotelemetry was considered the solution to the data-retrieval problem by scientists such as Arthur Compton in the 

United States, and Sergei Vernov in the U.S.S.R. In 1934, with 

Compton's support, J.M. Benade and R.L. Doan, at the University of Chicago, conducted a test flight of the transmitter portion of a sonde that contained an ionization chamber for measuring cosmic rays (Benade and Doan, 1935). In July 1934, 

Vemov designed and built a complete radiosonde that used two 

Geiger-Mueller (GM) tubes in a coincidence mode to measure 

the intensity and directionality of cosmic rays. Vernov 

flight-tested this instrument in an airplane in July 1934, and in 

April 1935, the instrument was flown as a balloon-borne radiosonde (Vemov, 1934, 1935). 

In Vernov's sonde, the coincidence output of the GM tubes 

activated a relay that switched on the anode circuit of a radio-frequency oscillator. At five-minute intervals the number 

of pulses from one of the GM tubes also was transmitted for a 

short period. Switching from coincidence output to a single 

GM tube was accomplished by means of a barometer switching 

device, so that the sequence of switching cycles indicated pressure (and altitude). This was the first radiosonde to measure 

cosmic rays, and its 1935 flight provided data at altitudes up to 

13.6 kilometers. In the same year, American scientist, T.H. 

Johnson, of the Bartol Research Foundation, flew a more elaborate cosmic-ray radiosonde to an altitude of about 20 kilometers, using coincidence counters (Johnson, 1937:352). The 

flights of Vernov and of Johnson provided some useful data, 

but their chief importance was to demonstrate the potential utility of the radiosonde for obtaining cosmic-ray data in remote 

regions, thus making possible the systematic survey of cosmicray intensity over a wide range of latitudes. 

In the 1930s, unmanned balloons were the only vehicles capable of reaching altitudes above 22 kilometers; however, because of the prolonged "float" and consequent drifting, the 

probability of recovering balloonsondes decreased as the maximum altitude increased. Thus, when radiotelemetry eliminated 

the data-retrieval problem inherent in the use of self-registering 

balloonsondes, the radiosonde became a valuable tool for research at very high altitudes. 

One factor investigated was solar radiation. In order to minimize the effect of the atmosphere, it was desirable to obtain solar flux data at the highest possible altitudes. Such research 

was begun by Charles Abbot in 1913, using a self-registering 

balloonsonde (see section 1, "Nontelemetering Balloonsondes: 
58 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

1892-1929") (Abbot et al., 1915) and was continued in the 

United States in 1937 by Brian O'Brien, using a radiosonde. In 

O'Brien's radiosonde, the light received by a quartz diffuser 

was passed through a filter that allowed light with the desired 

spectral characteristics to impinge on a vacuum photocell. The 

photo current thus generated discharged a capacitance that, 

upon recharge, caused a radio to transmit a short signal. The 

rate at which signal pulses were transmitted was proportional 

to the light incident on the photocell. Two photocell assemblies were used to cover the violet and red portions of the spectrum. In 1937, flights of this radiosonde provided measurements of solar flux to altitudes of about 22 kilometers, with a 

precision of about one percent. The altitude was determined 

visually by using a theodolite method (O'Brien et al., 1937). 

Later flights, involving a comparison of radiosonde data with 

measurements made simultaneously by ground-based instruments, yielded information on the absolute energy distribution 

of the solar spectrum (O'Brien, 1939). 

Reaching very high altitudes also was important in ascertaining, by extrapolation, the ultraviolet intensity of sunlight beyond the atmosphere. This problem was addressed by W. 

Coblentz and R.S. Stair at the U.S. National Bureau of Standards, using radiosondes. In the early 1930s, Coblentz developed an ultraviolet-intensity meter based upon a cadmium 

cathode photocell that was sensitive to radiation in the range of 

2600 to 3250 Angstroms. This meter had been used successfully at ground stations to measure ultraviolet radiation and, in 

1937, it was modified for use in a radiosonde (Stair and 

Coblentz, 1938:187). The amplifier and microammeter of the 

ground-based unit were replaced with a small audio-frequency 

generator, amplifier, and transmitter that had been designed for 

the NBS-Navy meteorological radiosonde. The output of the 

photocell modulated the transmitted audio frequency by changing the voltage on the generator control grid that, in turn, altered the frequency of audio oscillation. The photocell was exposed to sunlight that passed through a series of filters mounted 

on a rotating wheel. The photocell thus provided sequential 

measurements of the ultraviolet spectrum. Several flights of 

this radiosonde were made in the summer of 1937. 

Because ozone absorbs ultraviolet radiation, measurement of 

the ultraviolet intensity at various wavelengths as functions of 

altitude can be used to determine the concentration of ozone at 

specific levels of the atmosphere. Stair and Coblentz made additional flights in 1938 with an improved radiosonde to obtain 

ultraviolet measurements that could be used for determining 

ozone concentrations. These flights reached altitudes of 27 kilometers and ascertained that the ozone layer extended from 18 

to 27 kilometers, with the maximum concentration at 25 kilometers (Coblentz and Stair, 1939:573-576). These results confirmed data that had been obtained in the early 1930s by German scientist, Erich Regener, using a balloon-borne, 

self-registering, ultraviolet spectrometer (Regener and Regener, 1934. 

In 1939, cosmic-ray radiosondes were flown by scientists in 

several countries. In the United States, L. Curtis and A. Astin 

of NBS measured cosmic rays with a modified version of the 

NBS-U.S. Navy meteorological sonde. Belgium scientist, M. 

Cosyns, and British researchers, H. Carmichael and E. Dymond, also made flights using cosmic-ray radiosondes of their 

own design. The flights of Carmichael and Dymond were especially noteworthy because they provided the first vertical 

soundings of cosmic-ray intensities near the North Pole (Carmichael and Dymond, 1939:323-327). 

The polar regions were of great interest to researchers because they realized that theoretical understanding of cosmic 

rays would be advanced by measuring cosmic-ray intensity 

over a wide range of geomagnetic latitudes. The sonde developed by Carmichael and Dymond incorporated several GM 

tubes operated in a coincidence mode to obtain information 

about the intensity and directionality of cosmic rays. The coincidence output was relayed by the transmitter as a sequence of 

short pulses. The altitude was derived from pressure measurements, using the method utilized in the Vaisala meteorological 

radiosonde—an aneroid barometer that varied the spacing of 

the plates of a capacitance in the tuning circuit of the oscillator 

of the transmitter, thus altering the radio frequency emitted. In 

flights made at 85 degrees north latitude, cosmic-ray measurements were obtained at altitudes up to 27 kilometers. 

By 1940, the utility of research radiosondes to obtain useful 

geophysical data from high altitudes had been demonstrated. 

With the exception of the British polar flights, however, which 

supplied hitherto unavailable data on cosmic-ray intensity in 

that region, the data supplied by radiosondes had, for the most 

part, merely confirmed results on ultraviolet light, ozone, etc., 

previously gathered with self-registering balloonsondes. As described in the next section, the availability in the 1940s of a 

practical plastic balloon, capable of reaching heights above 45 

kilometers, enhanced the ability of the radiosonde to provide 

new geophysical data. Consequently, the special sondes of the 

1930s became the first in a long succession of research radiosondes that were built to explore the outer reaches of the atmosphere. 
7. The Radiosonde after 1940 

We have referred to the years 1929-1940 as the period of radiosonde development because the basic designs of meteorological radiosondes did not change radically after 1940. Research in the earlier period also demonstrated the utility of the 

radiosonde for a variety of geophysical applications, which is 

discussed in this section. 

One of the earliest problems of generating radio signals was 

frequency control. The defects of a transmitter whose frequency wandered were obscured to some degree by similar defects in receivers. While Blair, Bureau, Vaisala, Duckert, and 

others struggled with problems of reducing the spurious influences of temperature, humidity, and pressure on their transmitters, so that the calibrated sensors could give accurate readings, 

improvements in the selectivity of radio reception also made 

the problem of transmission crucial. Duckert, especially, was 

concerned with this problem because he was intentionally 

modulating the transmitter frequency to carry the sensor readings. The desired improvement was achieved by engineers 

working in other areas of radio transmission, who devised a 

means of using the extremely stable properties of quartz crystals to control the frequency of a radio-frequency oscillator. 

The use of quartz crystals to alleviate transmission problems 

was never widespread in radiosonde design. Radiosonde designers must have been aware of quartz-crystal circuits since 

the early 1920s, but they consistently rejected them in favor of 

simpler radio-frequency oscillators that used less precise methods of stabilization. The reason for this probably was the primary requirements for sonde design—simplicity and low 

weight. Quartz-crystal frequency control, although elegant and 

stable, required at least twice the number of components for an 

equivalent output signal power level. Not until the 1960s, when 

transistors came into common use in electronic circuitry, did 

the cost and weight effect of this penalty became less significant. 

The post-World War II period brought changes in the design 

of meteorological radiosondes, although they can perhaps be 

called evolutionary; that is, whereas engineering modifications 

were made periodically, in order to exploit advances in electronics and related technologies and to accommodate regulatory requirements of radio spectrum usage, the objectives of 

meteorological radiosonde design and the methods for achieving these objectives remained relatively unchanged (Dymond, 

1947:645-650). 

On the other hand, a different situation existed in the case of 

research radiosondes, where the objectives included the measurement of ozone, cosmic rays, etc., and involved exploitation 

of the dramatic advances in balloon technology in the 1940s. 

At the same time, designers sought to exploit new techniques 

of multichannel telemetry. Unlike the 1930s, when scientists 

constructed research sondes by borrowing ideas from the designers of meteorological radiosondes, the increasingly disparate goals of geophysical research and radiosonde design in the 

post-war world occasioned little of this kind of interaction. 

METEOROLOGICAL RADIOSONDES 

By 1940, the balloonsonde had been abandoned, and the radiosonde had been adopted by national weather services. The 

original designs were used for decades, and some are still used 

today, necessarily changed over the years to incorporate new 

electronic components and to conform to international regulations controlling the assigned frequency at which transmitters 

could operate. In general, these changes achieved improved 

performance at lower cost. 

Chronometric Sondes 

The Bureau sonde in France and the Jacobsen sonde in Canada were re-engineered periodically to include up-to-date electronic parts. In 1948, the Swiss weather service adopted a chronometric sonde designed by J. Lugeon. This mechanism 

utilized a clock-driven rotating assembly of the Olland type, 

but it incorporated a feature that minimized mechanical contact. The sensor arms were terminated in small plates that acted 

as capacitor plates when positioned above similar plates on the 

end of the rotating arm. Thus, the position of the arm was indicated by a change in capacitance that altered the transmitted 

signal (Lugeon et al., 1948:76). 

Coding Sondes 

The Moltchanoff sonde was progressively redesigned, not 

only to incorporate new electronic components, but also to improve the mechanics of the coding system. A version that appeared in the 1950s used a simplified coding mechanism consisting of a series of studs arranged in three arcs on an 

insulating plate over which the arms of the mechanical sensors 

moved. The position of a given arm was indicated by a windmill-driven mechanism that caused the transmitter to send an 

identifying code (Anonymous, 1961:101). Coding sondes also 

were developed in the early 1950s in Japan by K. Isono, and in 

Germany by H. Hinzpeter (Isono and Huziwara, 1950; Hinzpeter, 1951). A significant feature of the Japanese sonde was 

the use of two mercury-filled glass thermometers that eliminated the need for a preflight check of the temperature calibra59 
60 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 55.—A Japanese coding radiosonde of the type designed by K.. Isono. In this sonde a small electric motor 

revolved a raster of contacts so that it successively touched points moved by an aneroid barometer and by a 

bimetallic thermometer and hair hygrometer. A commutator switched between these meters and also made contact with two mercury thermometers, one of which disconnected the humidity contact at a temperature of -30° 

centigrade, the other stopped the sending of a special signal at -50° centigrade. Otherwise, the indications were 

interpreted by noting the sequence of Morse letters N, D, B, T, M, U. The transmitter, which operated on a frequency of 402 MHz, was powered by a small storage battery. The instrument is in a white cardboard container, 

27x15x11 cm, which acted as a radiation shield. (Instrument in NMAH collection, catalog number MHT 

319826; Smithsonian photo 65996.) 

tion (Figure 55). The sonde designed by Hinzpeter (Figure 56) 

was basically a redesign of Gaw's coding sonde and used new 

electronic components and a more reliable coding mechanism. 

Radio-Frequency Modulation 

Over the years, improvements were made in Finland to the 

Vaisala sonde. A model introduced in 1957 used a unique system to eliminate the effects of temperature on sonde components. The aneroid barometer, transmitter, reference capacitors, 

and switching mechanism were enclosed in a double-walled 

housing. Before launching, the space between the walls was 

filled with water near zero degrees centigrade. This water temperature was maintained throughout the ascent, despite de-
NUMBER 53 61 

FIGURE 56.—A Gaw coding sonde, as redesigned by H. Hinzpeter in 1950. The mechanical parts and transmitter 

are enclosed in foil-coated, fiberboard box about 1 Ox 10x20 cm. Metal shields for the hair hygrometer and the 

bimetallic thermometer are on the left and right sides of the central box, respectively. In addition to their complexity and relatively high cost, the design principles of coding sondes made necessary the employment of sensors whose output was a mechanical displacement. This precluded any simple incorporation of improved sensors, whose output was a change in some electrical characteristic that were developed in the 1930s. (Instrument 

in NMAH collection, catalog number MHT 324241; Smithsonian photo 61906-C.) 

creasing air temperatures, by the latent heat of ice formation 

(Anonymous, 1961:104-105). In Germany, a frequency-modulation sonde was developed by K. Sittel in the early 1940s and 

was used during and after World War II. Its chief feature was 

the elimination of mechanical linkages to the temperature sensor by using a capacitor having a temperature-sensitive dielectric as a thermometer. This method was similar to that used in 

the Duckert sonde described in the previous section. Indeed, 

the Sittel sonde (Figure 57) can be regarded as a redesign of 

the Duckert sonde (Sittel and Menzer, 1950). 

Audio-Frequency Subcarrier Modulation 

Radiosondes used by the weather services of the United 

States and Britain, which utilized audio-frequency subcarrier 

modulation, did not undergo any radical redesign after 1940, 

although many models were made. In the United States, the 

NBS-Navy sonde was adopted by the Weather Bureau, but 

during the early 1940s the U.S. Weather Bureau Model (Figure 58) continued to use the hair hygrometer instead of the 

electrical type used in some Navy versions (Snyder and 

Bragaw, 1986:124). In 1954, the Dutch Weather Service 
62 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 57.—A German radiosonde of the type designed by K. Sittel in which temperature and humidity were 

determined by a psychrometer. A pressure element and two thermometers—one covered with muslin—were 

used to vary the frequency of the signal. An electric motor was used to operate a switch that changed the connections among the sensors and two reference capacitors. These, as well as other parts of the transmitter, are contained in a Dewar flask. The entire instrument, except the thermometers, fits into a cylindrical aluminum case, 30 

cm long and 8.4 cm in diameter. The thermometers are in an attached triple shield, consisting of an outer oval 

cylinder. (Instrument in NMAH collection, catalog number MHT 314513; Smithsonian photo 61837-E.) 

adopted an audio-subcarrier sonde designed by Dutch scientist 

A. Hauer (Hauer and van Tol, 1954), which minimized the 

number of moving parts by adopting the same method used by 

the NPL in 1938, in which the output from three sensors was 

represented simultaneously. Three oscillators were required, 

but by 1954 the availability of subminiature vacuum tubes 

made the weight penalty of this device less than that of the 

earlier NPL design (Hauer and van Tol, 1954). In the late 

1950s, the cardboard case of the U.S. Weather Bureau sonde 

was replaced by a plastic case, and in 1960 transistorized radio 

circuits were introduced (Figure 59). By that time, the transmitting frequency of United States sondes had been increased, 

first to 400, and then to 1680 megahertz, chiefly to facilitate 

direction-finding. In 1960, transponders were introduced in 

some United States models (Figure 60) (Snyder and Bragaw, 

1986:131). 

The American production model of the 1950s, designated 

the AN/AMT-4, was a frequency-modulated audio subcarrier 

type that used amplitude modulation to impress the subcarrier 

onto the radio-frequency carrier signal. The radio-frequency 

oscillator was a small "pencil" triode operating in a cavity resonator and producing about one-half watt of RF power. At 

1680 MHz the antenna was small enough to be self-contained 

in the radiosonde package. The audio subcarrier was produced 

by a blocking oscillator modulated in the range from 10 to 200 

hertz by sensor and reference resistors. These sensors were 

switched into the modulator circuit in a sequence determined 

by a NBS baroswitch. Ground equipment automatically received the signal, demodulated the data, and recorded the separate variables on a chart recorder. Later in the decade, the design evolved further, receiving successive model designations 

AN/AMT4A, AN/AMT4B, etc., but retaining the same basic 

design (Anonymous, 1961:102). 

In the late 1950s and 1960s, additional models appeared 

with changes in mechanical construction, such as the AN/ 

AMT11 with a molded plastic case (Figure 59). Other models 

were designed for special purposes, such as the AN/AMQ-9, a 

transponding radiosonde (Figure 60). In this type of sonde, a 

radio receiver picked up signals transmitted from the ground 

at 403 MHz with pulse modulation at 75 KHz. Meteorological 

data were added when this signal was received and retransmit-
NUMBER 53 63 

FIGURE 58.—A U.S. Weather Bureau radiosonde (left), circa 1942, based upon 

a NBS-Navy design, is housed in a foil-covered cardboard case, 22x11x22 

cm. Two flaps open to reveal an electrolytic thermometer and a cylindrical 

array of hairs that operated a variable resistor. The hygrometer is mounted on 

white plastic molding and is enclosed in a semicylindrical white shield with 

louvers at the upper end. Batteries, a transmitter, and a "baroswitch" occupy 

the remainder of the carton. A similar sonde is shown (below) with the case cut 

away to allow most of the components to be seen. (Both instruments in NMAH 

collection, catalog number MHT 312798, above; catalog number MHT 

322315, below; Smithsonian photos 61905-B and 61905, respectively.) 
64 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

FIGURE 59.—A U.S. Weather Bureau radiosonde, circa 1960. This unit is housed in a white plastic box, 12x11.5 

x9 cm, and contains a solid-state transmitter and a baroswitch and has brass arms for supporting a thermistor. 

This radiosonde model transmitted at 1680 MHz frequency and has been in use since the mid-1950s. There is a 

provision for attaching a battery box. The instrument is marked "Radiosonde set AN/AMT1 IB." (Instrument in 

NMAH collection, catalog number MHT 324240; Smithsonian photo 61908-C.) 

ted back to the ground at 1680 MHz. This system provided a 

high-data rate from specific locations and altitudes. 

The most recent developments in meteorological sondes 

have incorporated the latest advances in electronics and sensor 

technology. The trend to higher frequencies, ending with the 

current international allocation at 1680 megahertz, began during the evolution of radiosonde prototypes in the 1930s. This 

trend was primarily caused by the need to reduce the size and 

weight of the devices. Early in the 1940s, a second factor, interference, began to influence the operating frequency of radiosondes. Substantial problems arose from other users of that 

portion of the radio band in which radiosondes then operated. 

Ordnance fuses, homing beacons, and communication devices 

exacerbated the frequency allocation problem. In 1946, the 

U.S. Federal Communications Commission, working within 

the framework of an international treaty, allocated a new frequency band, 400 to 406 MHz (71 cm), for radiosonde use. In 

the United States, the National Bureau of Standards modified 

its standard design to operate in this range and added an improvement in modulation technique. The latter was pulse modulation, which provided greater accuracy in recovering meteorological data. Radiosonde transmitter wavelength remained 

in the 71 centimeter range for about a decade before moving to 

the current allocation, in the microwave region, at 17 centimeters. 

The evolution of the audio-subcarrier frequency-modulation 

radiosonde was important to the development of data telemetry. The rapid advance of aviation technology in the late 1920s 

and 1930s required data transmission from aircraft in flight; 

measurements of pressure, temperature, electrical current, etc., 

relating to performance. Until World War II, these quantities 

were recorded on board the aircraft, using instrumentation 

similar to early balloon meteorographs. Eventually, however, 

more immediate readout of the data was required, especially in 

drone or unmanned test flights where there was a possibility 

that the vehicle would be damaged or destroyed. The radiosonde provided a well-developed technology that could be 

adapted for this specific task, transmitting analog electrical 

and (by transducers) mechanical data over a radio channel 

from air to ground. There was, however, a problem with 

straightforward adaptation of radiosonde technology to transmission of data from airplanes to the ground—there was too 
NUMBER 53 65 

FIGURE 60.—A U.S. Weather Bureau radiosonde with transponder, circa 1965. The plastic case measures 20x 14 

x 15 cm and has a tube projecting from the bottom containing a 1680-MHz transmitter and an antenna. The main 

case contains space for batteries, a clock-driven switch, and a receiver, which received signals from the ground at 

403 MHz, pulse-modulated at 75 KHz, and added audio-frequency pulses from meteorological instruments. The 

output was used to modulate the 1680-MHz transmitter. (Instrument in NMAH collection, catalog number MHT 

322310; Smithsonian photo 61905-C.) 

much data. Whereas only three variables (temperature, pressure, and humidity) were sufficient to satisfy the needs of the 

meteorologist using weather balloons, many more variables 

were needed by the aircraft designer. Even when a single variable, e.g., pressure, was of interest, aviation engineers wanted 

to measure it at numerous points. Existing radiosonde designs 

were not equipped to do this. Some unusual attempts were 

made to overcome this limitation. For example, in World War 

II, aviation engineers in the United States mounted a television 

camera in the cockpit to return data by transmitting pictures of 

a large panel of instrument indicators. Such methods of multiple-channel data transmission were generally unsatisfactory, 

however, and a more viable technology was needed. 

The engineering history of radiosonde development, however, contained a straightforward solution to the multiplechannel data-transmission problem. In 1938, the British NPL 

expanded the frequency-modulation audio-subcarrier radiosonde to three channels by the simple innovation of using 

three simultaneous audio frequencies. Each of these audio frequencies was frequency modulated independently by separate 

data (Thomas, 1938). The underlying theory of radio carrier 

modulation was highly developed by 1938. (Theory showed 

the possibility of complex modulation schemes far beyond the 

existing technology.) It was obvious that three or more audio 

subcarriers could be modulated simultaneously onto a radio 

carrier frequency without mutual interference. Although the 

British NPL radiosonde was unsuitable for balloon meteorology because of its weight and complexity, it was important as 

the prototype of a class of instruments used later to solve the 

problem of massive data retrieval in real time from aircraft 

and rockets. Multichannel data-telemetry systems, using multiple frequencies, evolved in the late 1940s from this radiosonde technology and became an entirely separate discipline. 

RESEARCH RADIOSONDES 

In the 1940s, the introduction of multichannel radiotelemetry made feasible the simultaneous transmission of the output 

of numerous sensors. The development of the plastic balloon 

also made it possible to loft payloads to altitudes above 45 kilometers. It is noteworthy that these two developments had little effect on meteorological radiosondes because neither the 

simultaneous transmission of various measurements nor the 

achievement of such great altitudes were needed by users of 

such sondes. These advances in radiotelemetry and balloon 

technology did, however, have a profound influence on the designs of research radiosondes. 

Methods of multichannel radio transmission via subcarrier 

modulation were developed during World War II for various 

military purposes. Also, the development of multichannel radio telemetry was accelerated in the immediate postwar years 

by the needs of military rocket programs. Multichannel telem-
66 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

etry was quickly adopted for research sondes because the capability to transmit simultaneously the output from several sensors was an important consideration in most research 

applications. For example, cosmic-ray studies usually involved 

the correlation of atmospheric density with cosmic-ray data. 

Such correlation was facilitated by obtaining simultaneous outputs from cosmic-ray detectors and from pressure and temperature sensors. Also, in most investigations involving research radiosondes attempts were made to maximize measurement 

accuracy. Thus, the availability of extra channels to transmit 

the output of reference standards contributed to the precision of 

the data. Moreover, the reliability of research sondes was improved by a multichannel capability that allowed redundant 

sensors to be included in payload designs (Borden and MayoWells, 1959:204-268). 

Postwar developments in balloon technology influenced the 

design of research radiosondes by making it possible to reach 

altitudes hitherto unexplored by instrumented probes. The origins of this new balloon technology began with the efforts in 

the early 1930s of German scientist Erich Regener to measure 

cosmic rays at very high altitudes, using self-registering balloonsondes (Millikan and Bowen, 1926; Regener, 1932). At 

that time the question of whether "primary" cosmic rays (i.e., 

the component of such radiation existing outside the earth's atmosphere) were electromagnetic waves or charged particles 

was still unresolved. Measurements at the highest altitudes attainable could provide information regarding this question, 

hence, Regener sought design improvements that would allow 

his balloons to achieve higher altitudes. 

It was known that ozone weakened rubber and that rubber 

balloons usually burst below their maximum theoretical altitude. Regener surmised that the failure of rubber sounding balloons to reach altitudes much greater than 27 kilometers was 

attributable to the high concentration of ozone between 18 and 

27 kilometers. He reasoned that, as balloons passed through the 

ozone layer, their skin was chemically attacked, causing the 

balloon to rupture at the weakest point (Regener, 1935). The 

solution to this problem was to use an ozone-resistant material. 

In the 1930s, cellophane was the only material available for 

this purpose, but it lacked flexibility and had undesirable 

low-temperature characteristics that made it poor material for 

balloon envelopes. Nevertheless, cellophane balloons were 

constructed and flown by Regener, probably as early as 1934 . 

In 1935, an American scientist, T.H. Johnson, lofted a radiosonde using three cellophane balloons. Johnson's sonde (as discussed in the previous section) was designed to measure cosmic rays. On its first flight it provided data to about 20 

kilometers. Johnson had constructed the balloons with the help 

of another scientist, Jean Piccard (twin brother of August Piccard, who made the first manned balloon flight into the stratosphere in 1931). In the late 1930s, Jean Piccard conducted additional research on cellophane balloons at the University of 

Minnesota. The cellophane balloons of the 1930s still did not 

reach altitudes greater than those attained by rubber balloons 

because of the unsuitability of cellophane as a balloon material 

(Pfotzer, 1972:212-226). 

After World War II, Piccard, in association with Otto Winzen, an engineer, persuaded the American firm of General 

Mills to make polyethelene plastic balloons. The Office of Naval Research (ONR) was sufficiently impressed with the research potential of such balloons that they provided the initial 

funding to General Mills for the development of a plastic balloon capable of reaching very high altitudes. As a result of the 

ONR program, the first "Skyhook" balloon was built and flown 

in September 1947, reaching an altitude of about 30 kilometers. 

This new technology produced balloons capable of lofting instrument payloads weighing many pounds to altitudes above 45 

kilometers, which is above 99.9 percent of the atmosphere surrounding the earth (Pfotzer, 1972:227-231). 

During the last four decades, research radiosondes have provided a wealth of information about cosmic rays, atmospheric 

composition, ultraviolet radiation, and solar flux. Despite the 

advent of rockets and satellites, balloon-borne research radiosondes still are used because of their unique advantages, relative to other vehicles, primarily low cost and the ability to remain at a given altitude for long periods. Thus, even in the 

space age, research radiosondes are still a useful investigative 

tool. Some aspects of rocket flight even depend upon the radiosonde. Winds aloft between about 15 and 45 kilometers must 

be measured with some precision just before the launch of most 

types of rocket to insure safe travel through this altitude, where 

aerodynamic stresses on the vehicle are greatest. It is costly 

(and for the upper part of this altitude range impossible) to fly 

airplanes at that altitude to obtain measurements. Consequently, radiosondes are flown prior to virtually all rocket 

flights to measure wind velocities and other atmospheric parameters that could affect the rocket's ascent. Often launch 

countdowns are put on "hold" because of adverse radiosonde 

data. 
8. Conclusion 

The improvement of agriculture, a paramount economic global concern, was the main objective of meteorology through 

the nineteenth century. By the end of that century, the art of 

weather forecasting had made significant progress. Moving air 

masses were recognized as the carriers of local weather, and 

data about wind speed and direction, pressure, temperature, and 

water content (humidity), in one location, were used to forecast 

the weather elsewhere. It was an imperfect art, and the farmers 

continued to have more confidence in the Farmer's Almanac. 

In the early twentieth century, the emerging technology of 

aviation provided an additional and urgent need for more accurate weather forecasting. This need assured greater material 

support for meteorologists, including government financing, 

because of the obvious military value of aircraft. 

The traditional claim of the meteorologist, that the weakness 

of this science stemmed from insufficient data, was reinforced 

when the needs of aviation emphasized the limited utility of 

measurements made from ground level. Unmanned kites and 

captive balloons were used for decades, but these were limited 

to relatively low altitude and could not be used in many locations for safety reasons. Observation stations on the tops of 

mountains and hills provided some limited data. Observations 

also were made from airplanes, but this was a "chicken and 

egg" situation—the flights could not be widespread and safe 

without first having the results of the observations they would 

be making. As late as 1920, there were very few ways of collecting data from altitudes above a few hundred meters, and no 

means existed for collecting such data across wide areas in an 

organized fashion. The advent of heavier-than-air-flying machines worsened the problem because it required the collection 

of meteorological data at high altitudes on a timely basis. The 

available technology did not offer the means to satisfy these requirements until the invention of the wireless telegraph (radio). 

Marconi's transmission of radio signals across the Atlantic, in 

1901, provided a solution to the problem of timely collection of 

meteorological data, which was used three decades later. 

The speed with which these components were put together to 

create the radiosonde was remarkable. It hardly involved any 

advance in science, but rather the appearance of an urgent need, 

aviation. Nor was technological innovation very conspicuous. 

Important factors were the concurrent emergence of a popular 

hobby—amateur radio—and the commercial availability of 

vacuum tubes and other components. Finally, there was that 

never-failing stimulus to development—military application. 

All this seems obvious now. The successful development of 

the radiosonde tends to hide the complexity of the research by 

which it was produced. The radiosonde represented one of the 

first comprehensive applications of radio to a purpose other 

than communication. This use of radio technology to meet a remote-sensing requirement was not unique, but it did set a precedent that accounts for some aspects of the history of subsequent instrumentation. 

There also were important consequences for the then backward science of meteorology. Thanks to data provided by the 

radiosonde from a range of altitudes, synoptic weather maps 

were vastly improved. These data, in turn, provided the means 

to generate timely, accurate forecasts based upon the motion 

and evolution of the air masses. As radiosonde technology and 

data collection improved in the 1940s, scientific meteorology 

finally matured. Deterministic modeling of the atmosphere, 

based upon the physical laws of gas dynamics and heat transfer, 

although appropriate, had long been considered futile because 

measurements on a sufficiently large scale and at high enough 

resolution to establish initial conditions for the equations could 

not be made. The availability of large amounts of data from radiosondes and the emergence of electronic computers in the late 

1940s helped to forge a new branch of science in practical modeling of the atmosphere. Modeling, together with skillful interpretation of data, has promoted a steady improvement in our 

understanding of the atmosphere and its dynamics. 

From the perspective of the history of technology, the radiosonde also served as an important link in the development of 

multichannel radio transmission, using subcarrier modulation. 

Because this became the basis of modern analog radiotelemetry 

systems, the development of the radiosonde represented an important phase in the history of telemetry. 

Ground observations continue to be important, but they provide only a small portion of the necessary data. Atmospheric 

data collection by satellite, starting with the NIMBUS and 

TIROS programs in the 1960s and 1970s, is now supplementing radiosonde data. Satellites have not yet completely replaced 

the radiosonde, however, because of the difficulty of obtaining 

vertical measurements from the overhead view of a satellite. 

Techniques to accomplish high-resolution vertical profiling of 

atmospheric temperature by infrared radiotelemetry have been 

refined in successive generations of TIROS (now called 

NOAA) and other weather satellites. Additional parameters, 

such as wind vectors and barometric pressure, also can be determined from satellites, but to a lesser degree of precision than 

by radiosondes. Nevertheless, it is likely that, by some time in 

the twenty-first century, the majority of radiosonde data collection will be replaced by satellite-derived measurements, and radiosonde deployment will be relegated to a minor, niche technique. 

67 
III. Catalog of Upper-Atmospheric Telemetering Probes 

and Related Artifacts in the National Museum 

of American History, Smithsonian Institution 

Robert P. Multhauf and George A. Norton, Jr. 

Catalog 

number 

Accession 

number 

Figure 

number Type Identification Date Donor 

312798 162632 58 USWB radiosonde 1942 Plaskon Company 

Marked "Sonde Track... Mfg. Washington Institute of Technology. USWB 1942." This foil-covered cardboard box, 22x21.6x12 

cm, contains sensing elements for measuring temperature (electrolytic), humidity (hair hygrometer), a baroswitch, and a transmitter. Received from the Plaskon Company, 2121 Sylvan Ave., Toledo Ohio, in March 1944. 

312799 162632 54 NBS-U.S. Navy hair hygrometer 1942 Plaskon Company 

Marked "WIT." Aneroid element, 9x5.5x20.3 cm. This is the same type used in item 312798 and was received at the same time 

from the same donor. 

312800 162632 52 NBS-U.S. Navy baroswitch 1942 Plaskon Company 

Marked "WIT." Aneroid barometer, 13x9x6 cm. This type was used to control the selector switch in item 312798, and it was received at the same time from the same donor. See text, pages 50-54. 

312801 162632 NBS-U.S. Navy record of data 1942 Plaskon Company 

Photostatic copy of data, pressure, temperature, and humidity, transmitted by item 312798 at Washington National Airport, 22 

November 1941. Received at the same time from the same donor as item 312798. 71x11.8 cm. 

312802 162632 NBS-U.S. Navy transmitter 1942 Plaskon Company 

Radio transmitter in plastic case, 7.7x9x5.2 cm. This type was used in item 312798 and was received at the same time from the 

same donor. 

313527 188571 35 Duckert radiosonde ca. 1932 NBS 

Unmarked except for "NBS no. 0203." This clear plastic cover and aluminum frame, 19x9x31 cm, contains a vacuum tube 

within which are located the transmitter, a toothed wheel switch actuated by a Bourdon tube (also the pressure element), and a bimetallic temperature element. Received from NBS in September 1950. For information on Duckert's radiosondes, see text, pages 

34-40, and E. Kleinschmidt, 1935:282. 

313528 188571 Diamond et al. radiosonde 1936 NBS 

Unmarked balsawood box with externally mounted photo tubes, overall 22x 15x 17 cm. The box contains a transmitter (three vacuum tubes), a barometric switch (aneroid), and temperature (sulphuric acid resistor) and humidity (hair hygrometer) measuring elements. Identified by the donor as the first experimental radiosonde developed by H. Diamond, W.S. Hinman, Jr., and F.W. Dunmore at NBS and first used on 25 December 1936. A mark on the frame indicates that it was made by Friez & Sons of Baltimore. 

Received from NBS in September 1950. 

313529 188571 - NBS transmitter - NBS 

Unmarked box containing parts relating to radio transmission. This experimental apparatus was received with item 313528. 

314513 204101 57 Vaisala, Schulze radiosonde 1942 K.M.Perry 

Marked "493." This aluminum case, 11 x 15x30 cm, with its lower section painted black, contains elements for measuring temperature (a ceramic, temperature sensitive dielectric), pressure (aneroid barometer controlling a condenser), and humidity (wet 

and dry bulb). Identified by the donor as being of the Vaisala type, improved by Schulze at the "Marine Observatorium" at Griefswald (Germany) in 1942-1944. Received in November 1954. (See R.K. Sittle and E. Menzer, 1950:341.) 

68 
NUMBER 53 69 

Catalog Accession Figure 

number number number Type Identification Date Donor 

314514 204101 - unknown radiosonde K.M.Perry 

Marked "61426," and bearing a stamp indicating a German origin. The brown fibre case, 10.4x14.3x20 cm, has metal ends and 

contains bimetallic thermometric and aneroid barometric elements. Received in November 1954. 

314677 209614 USWB radiosonde 1955 USWB 

Marked "Northeastern Engineering, Inc.," with USWB serial no. 509401. The white cardboard carton (cut away), 20x23x 12 cm, 

contains a variable audio-frequency transmitter, a thermistor, and an electric hygrometer. This instrument was used on 13 July 

1955 and was found, after ascending to a height of 32 kilometers, at Redwater Alta, Saskatchewan, Canada. 

316262 224564 32 Moltchanoff radiosonde ca. 1935 USWB 

Marked "39097." The brown cardboard case, 7x27x33 cm, contains elements for measuring temperature (bimetallic), pressure 

(Bourdon tube), and humidity (hair hygrometer). It incorporates a rotating contact of the Olland type. The transmitter is missing. 

For information on Moltchanoff's instruments, see text, pages 33 and 34, and Lange, 1935:286. Received in March 1959. 

317728 231230 - USWB radiosonde 1960 USWB 

Marked "Ray Sonde Bendix." This blue and white cardboard box, 11.4x18x20.3 cm, contains elements for measuring temperature (electrolytic) and humidity (electrolytic), and a barometric switch. Received in June 1960 and identified by the donor as a 

contemporary instrument. 

319744 239017 NBS radiosonde 1940 E.L. Reilly 

Marked "Sonde Track. Mfg. in USA by Washington Institute of Technology," and the inspection date 9/25/40. The two nested 

cardboard boxes, 11x25.4x22 cm, enclose elements for measuring temperature (electrolytic), pressure (aneroid), and humidity 

(hair hygrometer), all reportedly developed at the NBS. Identified by the donor as WIT Sonde Track no. 1, produced on 25 September 1940, flown at Oklahoma City on 1 October, and recovered 13 October at Presque, Oklahoma. This is an example of the 

NBS variable audio-frequency radiosonde. Received November 1961. 

319824 240347 - (British) radiosonde 71937 USWB 

Marked "D698." Elements for measuring temperature (bimetallic), pressure (aneroid), and humidity (goldbeater's skin) project 

from the sides of a cardboard cylinder 13 cm in diameter by 14 cm long. Incomplete. Identified by the donor as a British instrument. Received in January 1962. (See Lange, 1937.) 

319825 240347 43 Vaisala radiosonde ca. 1950 USWB 

Marked "180140." The aluminum box, 9x31x18 cm, contains elements for measuring temperature (bimetallic), pressure (aneroid), and humidity (hair hygrometer), and incorporates a windmill-operated selector switch. (See text, pages 46^48, and item 

314513.) Received January 1962 and dated by the donor as being from about 1950. 

319826 240347 55 - Japanese radiosonde ca. 1960 USWB 

Marked "CMO-S50L" and stamped "DNPA No. 4552, Japan. Type Approval, Radio Regulatory Commission" (with additional 

label in Japanese). This white box of corrugated paper, 28x 12x 16 cm, contains elements for measuring temperature (two mercury and one bimetallic thermometers), pressure (aneroid), and humidity (hair hygrometer). Received in January 1962. 

319827 240347 30 Bureau radiosonde ca. 1932 USWB 

Marked "Constructions Mecaniques et Electriques P. de Presale, 104-106, Rue Oberkampf, Paris (XI) Type PTU (Sereal) 3567." 

The aluminum foil covered box, 27x 16x 14 cm, contains a transmitter (vacuum tube and batteries missing) and elements for measuring temperature (bimetallic), pressure (bourdon tube), and humidity (hair hygrometer). Temperature and pressure were indicated by interruptions in the transmitting signal. For information on Bureau's instruments see text, pages 31-36. Received in January 1962. 

319828 240347 - Cal. Inst. Tech. radiosonde ca. 1935 USWB 

Unmarked, but tagged by donor "Composite California Institute of Technology type." This instrument, 10x15x24 cm, contains 

elements for measuring temperature (bimetallic), pressure (aneroid), and humidity (hair hygrometer), and a clock-driven selector 

switch. Received January 1962. 
70 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

Catalog Accession Figure 

number number number Type Identification Date Donor 

319829 240347 - Lang radiosonde sensing elements - USWB 

Marked "10818." The metal frame, 12.7x 10x5.7 cm, contains elements for measuring temperature (bimetallic) and pressure (aneroid), and a clock-driven selector switch. Received January 1962 and identified by the donor. 

319830 240347 Astin, Curtis radiosonde sensing elements - USWB 

Unmarked. The metal frame, 19.4x8x9 cm, contains elements for measuring temperature (bimetallic), pressure (aneroid), and 

humidity (goldbeater's skin), plus an electrically driven switching disk. Received January 1962. (See also cat. no. 319833.) 

319831 240347 Blue Hill radiosonde ca. 1936 USWB 

Marked "RMF 84," and tagged by donor "original Blue Hill." The sensing elements, mounted on a 10x20x6 cm board, are for 

measuring temperature (bimetallic), pressure (aneroid [missing]), and humidity (hair hygrometer). Transmission was controlled 

by a clock-driven spiral. Transmitter and battery box are missing. Received January 1962. (See Lange, 1937:110.) 

319832 240347 - Friez radiosonde ca. 1936 USWB 

Unmarked, but tagged "Friez" (of Baltimore, assumed to be the maker). The octagonal wooden box, 16x 17x20 cm, contains elements for measuring temperature (bimetallic), pressure (aneroid), and humidity (hair hygrometer), plus a transmitter and a clock 

control for transmission. Received January 1962. 

319833 240347 Astin, Curtis radiosonde ca. 1938 USWB 

Unmarked. The cylindrical tube, 15 cm in diameter and 34 cm long, is made of corrugated paper, with a reflective painted surface. 

The tube houses the transmitter and clock. Projecting above is a housing containing sensing elements for measuring pressure (aneroid) and humidity (goldbeater's skin)—the temperature element is missing (but see cat. no. 319830). The unit was identified by 

the donor as an experimental instrument from the late 1930s. A.V. Astin was at the National Bureau of Standards from 1930 and 

was its director from 1952 to 1969. L.F. Curtis was at the Bureau from 1926. Received January 1962. 

322051 246537 ARCAS rocket sonde unk. ARC 

Marked "Radiosonde; Atlantic Research Corp., Alexandria, Va. Radiosonde AN/DMQ6 (XE-3)." The radiosonde, 11 cm diam., 

47 cm long, is an ARCAS high-altitude sounding rocket system. It contains four thermistors, which, with its rocket and aluminum 

parachute, measures a total length of 2.3 meters. Received January 1963. 

322308 247582 Simmonds radiosonde ca. 1942 USWB 

Marked "Simmonds chronometric radiosonde. P.B.I21." The pressed paper container, 24x 18x 17 cm, contains a blocking oscillator, an FM transmitter, and a clock-driven switch. Sensors for measuring temperature (bimetallic), pressure (aneroid), and humidity (hair hydrometer) are attached. The device was described by the donor as experimental. Received March 1963. 

322310 247582 60 U.S. Army radiosonde ca. 1956 USWB 

Marked "Radiosonde receiver AN/AMQ-9 (XE-3) Serial 201. File no. 360 PH-55-91 (2229) Standard Coil Products Inc. US." A 

white plastic case, 15x20x 14.6 cm, houses an FM blocking oscillator transmitter, a clock-driven selector switch, and sensing elements for measuring temperature (thermistor) and humidity (lithium chloride). The unit was identified by the donor as an instrument developed for the military establishment about 1956. Received March 1963. 

322311 247582 Lang radiosonde - USWB Mkd. 

Marked "W.S.E. 2 Gerat Nr. 124 75A Anf. Zeichen Ln 2879." This 16x31 x34 cm box has an attached plastic cylinder that contains a transmitter. The sensing elements are a bimetallic thermometer and an aneroid barometer; the element for measuring humidity is missing. It was identified by the donor as the Lang-Reichsamt fur Wetterdienst radiosonde. Received March 1963. (For 

information about the Lang radiosonde, see text, page 42.) 

322312 247582 (German) radiosonde ca. 1936 USWB 

Numbered "1054" and "712U"; otherwise unmarked. The aluminum-painted cardboard box, 8x12x17 cm, contains a bimetallic 

temperature-measuring element, an aneroid barometer, an electric motor with a star selector wheel, and a transmitter. It was identified by the donor as a "prewar" German instrument. Received March 1963. 
NUMBER 53 71 

Catalog Accession Figure 

Number Number Number Type Identification Date Donor 

322313 247582 46 British (Kew) radiosonde - USWB 

Marked "Whiteley Electrical Radio Co. Ltd., Mandfield Notts. England," and "Mark II Met. Ref. No. 280. II." The black molded-fiber cylinder, 22.5 cm long x 14 cm in diameter, contains a pulse modulation transmitter, sensing elements for measuring temperature 

(bimetallic), pressure (aneroid), and humidity (goldbeater's skin), and a windmill selector switch. Received March 1963. 

322314 247582 Diammond, Hinman radiosonde ca. 1943 USWB 

Marked "Diamond-Hinman Raysonde. Mfg. by Julien P. Friez & Sons, Baltimore, Md." The corrugated box, 21 x 12x30 cm, is 

covered with white paper and contains an FM blocking oscillator transmitter, a barometric selector switch, and sensing elements 

for measuring temperature (thermistor), pressure (aneroid), and humidity (electrolytic). Received March 1963. 

322315 247582 58 USWB radiosonde ca. 1942 USWB 

Marked "Sonde Track Washington Institute of Technology, Washington, D.C. Serial no. 214029." This corrugated, foil-covered 

box, 11x22x22 cm, contains an FM blocking oscillator transmitter, an aneroid baroswitch, and elements for measuring temperature (electrolytic) and humidity (hair hygrometer). Received March 1963. 

322316 247582 "Longine" radiosonde USWB 

Unmarked, but tagged "Longine." The aluminum frame, 18x 10x36 cm, houses a transmitter (FM blocking oscillator type), a 

clockwork selector, and elements for measuring pressure (aneroid), temperature (bimetallic), and humidity (hair hygrometer). Received March 1963. 

322320 247582 Ray Sonde radiosonde ca. 1939 USWB 

Marked "Olland System Ray Sonde. Julien P. Friez & Sons, Baltimore." The corrugated, foil-covered box, 22x 11 x21 cm, houses 

a transmitter (FM blocking oscillator type), an Olland switching system, and sensing elements for measuring pressure (aneroid) 

and humidity (hair hygrometer). There is no evidence of a temperature element. Received March 1963. 

322322 247582 Diamond, Hinman radiosonde ca. 1939 USWB 

Marked "Ray Sonde. Diamond-Hinman. Mfg. by Julien P. Friez & Sons of Baltimore," and stamped "Serial no. w2982" and "July 

1, 1939." The foil-covered corrugated box, 23x11x21 cm, contains a transmitter (FM blocking oscillator type), a barometric 

switch, and sensing elements for measuring temperature (electrolytic), pressure (aneroid), and humidity (hair hygrometer). 

322323 247582 - Friez radiosonde ca. 1940 USWB 

Marked "Meteorograph. Friez Baltimore." The foil-covered corrugated box, 11 x28x21 cm, contains a transmitter (FM blocking 

oscillator type) and sensing elements for measuring temperature (electrolytic), pressure (aneroid), and humidity (hair hygrometer). Received March 1963. 

322325 247582 48 Blue Hill radiosonde ca. 1936 USWB 

Marked "Fieber Instrument Co., Cambridge, Mass." The balsa wood box, 18x 10x7 cm, contains a transmitter (FM blocking oscillator type), and a clockwork selector switch. Sensing elements for measuring temperature (bimetallic) and humidity (hair hygrometer) are attached to the box. For information about Blue Hill radiosondes, see text, pages 50, 51 and item 319831.) Received March 1963. 

324236 254096 - U.S. Navy radiosonde USWB 

Marked "Radiosonde AN/AMT11... Molded Insulation Co., Serial 1675." The molded white plastic box, 12.7x 13.3x9 cm, contains a vacuum-tube transmitter, a barometric switch, and sensing elements for measuring temperature (thermistor) and pressure 

(aneroid). The humidity sensor is missing. This device was identified by the donor as having been made for the U.S. Navy. Received April 1964. 

324237 254096 - (Japanese) radiosonde ca. 1960 USWB 

Marked "CMO-S50L. Serial 4352." The white corrugated box, 12.4x29x 19 cm, contains a transmitter, a motor-driven drum selector switch, and sensing elements for measuring temperature (metallic expansion, plus a fixture for two mercury thermometers, 

which are missing), pressure (aneroid), and humidity (hair hygrometer). The unit was dated by the donor. Received April 1964. 
72 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

Catalog Accession Figure 

Number Number Number Type Identification Date Donor 

324238 254096 USWB radiosonde 1963 USWB 

Marked "USWB-Molded Insulation Co., Phila. Serial 37-00006," and stamped "Feb. 8, 1963." This corrugated, paper-covered 

box, 13x 16x 12 cm, contains a transmitter, a barometric switch, and sensing elements for measuring temperature (thermistor) and 

pressure (aneroid). The humidity sensor is missing. Received April 1964. 

324239 254096 (Japanese) radiosonde 1945 USWB 

Marked in Japanese, and with serial no. "020918." The white cylindrical plastic case, 16.5 cm longxll cm in diameter, contains 

sensing elements for measuring temperature (mercury thermometer [broken]), pressure (aneroid), and humidity (hair hygrometer), each with its own vacuum tube, circuitry, and antenna. The device was dated by the donor. Received April 1964. 

324240 254096 59 U.S. Navy radiosonde USWB 

Marked "Radiosonde set AN/AMT 11 B... Molded Insulation Co." The white plastic box, 12.4x13.7x10 cm, contains a solidstate transmitter, a barometric switch, and a sensing element for measuring temperature (thermistor). The humidity element is 

missing. This device was identified by the donor as one that was intended to be expendable. Received April 1964. 

324241 254096 56 Gaw (German) radiosonde 1963 USWB 

Marked "Transport Aufstieg. 5821." This foil-covered cardboard box, 1 Ox 10x20 cm, contains a transmitter, a motor-driven selector switch, and sensing elements for measuring temperature (bimetallic), pressure (aneroid), and humidity (hair hygrometer). Received April 1964. (For further information, see text, pages 60, 61.) 

324242 254096 - USWB thermistor ca. 1960 USWB 

Unmarked. This temperature measuring element consists of a wire coated with lead monoxide that is housed within a glass tube. 

This improvement in temperature measurement was developed at Johns Hopkins University about 1946 under contract with the 

USWB. (See Brasefield, 1948:145-151.) 

327975 254096 50 NBS-U.S. Navy thermometer/hygrometer 

The thermistor and electrolytic hygrometer assembly are mounted on a bakelite board. Overall dimensions are 11 x 12.7x74 cm. It 

was probably experimental. Received October 1966. 

Uncataloged - - USWB temperature element 1957 -

Unmarked but labeled "Temperature element ML-419/AMT-4. Molded Insulation Company, US," and stamped "Feb. 1957." 

This wire element in a glass vial was probably an experimental thermistor from the USWB, although the source is unknown. 

Uncataloged - USWB hygristor 1959 

The small can is stamped "USWB hygristor, electric hygrometer element. Model no. 524318-3. Bendix Aviation Corp., Friez Instrument Division, Baltimore, Md." and is dated 16 July 1959. It was probably from the USWB. 
Notes 

1. The hair hygrometer measures relative humidity in terms of 

dimensional changes on one or more human hairs stretched on a frame. It was 

invented in 1793 by French scientist Horace de Saussure, and by 1820 it had 

become widely used in meteorological research. By the 1890s, hair 

hygrometers were highly developed as sensors for humidity. 

2. Charles' balloon was filled with hydrogen. An alternative gas-fill 

appeared in the early 1800s when "coal gas" (chiefly a mixture of hydrogen 

and methane) began to be produced by municipal gas works for fueling the 

street lamps of many European cities. Coal gas was first used in a balloon in 

1821, and although it has only half the lifting power of hydrogen its much 

lower cost and ready availability made it the gas of choice for balloon use. The 

unmanned balloons used by Hermite and other meteorological investigators 

generally were filled with coal gas. 

3. Letter from Millikan to W. Blair, Millikan Papers, 28 November 1922, 

Roll 23, Folder 23.1. Library of the National Museum of American History, 

Smithsonian Institution. 

4. The concept of entrainment in a complex system actually predates 

Olland, Bain, telemetry, and computers by at least a billion years, having been 

worked out by Darwinian evolution on living organisms in response to the 

survival advantage of photoperiodic behavior. Virtually all living organisms 

exhibit a cyclic pattern of motor or biochemical behavior, which is entrained 

to the solar photoperiod. If such an organism is studied under conditions 

where environmental light and other geophysical parameters are held constant, 

the behavior cycle will usually continue at a period different from the 24-hour 

solar period. Under natural conditions the solar photoperiod resets the 

behavior cycle once per period, which synchronizes the sender (sun) with the 

receiver (organism). 

5. Radio wavelength is an important parameter. It determines, to a large 

degree, the size and weight of transmitter components as well as the antenna. 

Also, before World War I, radio-band usage was not regulated by governments or international treaty and, being in the same range as other users, there 

was considerable chance for interference. The earliest radiosondes of Blair and 

of Bureau adapted radio frequency transmission circuits directly from the 

technology of contemporary radio telegraphy and radio telephony. Blair's first 

transmitter used a wavelength of 125 meters (2.3 MHz) in 1924 (Blair and 

Lewis, 1931:1532) and Bureau's 1929 radiosonde operated at about 60 meters 

(4.8 MHz) (Bureau, 1931). 

The early investigators were aware of the potential advantages of shorter 

wavelengths, but they faced substantial technological barriers to using short 

waves. Transmitter circuitry and vacuum tubes that were small and would 

operate reliably at wavelengths below about 20 meters were not well developed 

in the mid-1930s. In spite of this, the advantages were great enough that, starting with Blair at 125 meters, there was an evolution toward the shorter wavelengths that continued though the 1950s to 17 centimeters (1680 MHz) where 

current radiosondes operate. This evolution can be traced through the first two 

decades of radiosonde development. First, Bureau settled on a shorter wavelength than Blair, 60 meters, and was able to shorten his antenna considerably. 

The United States program, starting in 1935 at the National Bureau of Standards (NBS), began at 4 meters (70 MHz), while Vaisala in Finland was using 

11 meters (26 MHz) at about the same time. Diamond, at NBS, then progressed 

to wavelengths below 2 meters for the "Explorer" stratosphere balloon project 

in 1935 (Snyder and Bragaw, 1986:102-104). 

A rapid decrease in wavelength followed as the technology of vacuum tubes 

and radio-frequency circuits for higher frequency advanced. The first production models of the NBS radiosonde were standardized in the 4-meter range 

because that wave length was low enough to achieve almost all of the advantages possible in smaller size and weight. It did not, however, place the radiosonde signals in a suitable part of the radio band to solve all of the problems 

resulting from signal interference. Crowding of the radio spectrum from overlapping transmitter frequencies had begun during the rapid expansion of radiotelegraphy before World War I, and the problem was serious enough to merit 

the formation of regular international conferences to create cooperative allocations of frequency bands for international use. The International Radiotelegraph Conferences were held in 1912, 1927, and 1932. The International 

Telecommunication Union (ITU), which survives to the present day, evolved 

during the 1932 meeting to become the principal international forum for these 

allocations. Radiosondes, which were undergoing their own evolution in the 

same period, were almost from their inception subject to the advantages as well 

as disadvantages of these allocations. The disadvantages lay in sharing of their 

spectrum with other services and consequent interference. The interference 

played a significant part in an evolution toward operating frequencies above 

100 MHz, where there was less usage, and pressure on the ITU for protected 

frequency allocations during the 1940s. 

6. The transition from discharge-excited radiation to the CW vacuum-rube 

oscillator may have occurred as early as 1921. A reference appears in a later 

historical review that is difficult to interpret. Duckert (1932:74) reported that 

"Already in 1921 Herath had tried to use an audio-oscillator made by Eichorn 

as transmitter in his balloon." The purpose, as with Herath's spark transmitter, 

was to track the balloon and to determine wind speed and direction. The 

success was reported as marginal, but the real curiosity is in the term "audiooscillator." If this is taken to mean a vacuum tube oscillator operating in the 

audio range, say 10 to 20 KHz, then it is truly the first use of a vacuum-tube 

CW oscillator transmitter from a balloon. 

7. Moltchanoffs assertion to have flown the first radiosonde is quoted in 

Beelitz (1954:69). This claim was echoed in some contemporary scientific 

journals, e.g., see the commentary on Moltchanoffs radiosonde by J.F.H. 

(1932:1006) and in some modern historical works, e.g., see Pfotzer (1972: 

242). Moreover, historians who recognized Bureau's priority over 

Moltchanoff, such as Middleton (1969a:329), appeared to be unaware of 

Blair's priority over Bureau. 

8. Mechanical sensors inherited from the balloonsonde were employed in 

the first radiosondes: the bimetallic thermometer, aneroid or Bourdon-tube 

barometer, and hair or goldbeater's skin hygrometer. The barometers suffered 

from hysteresis and tended to be temperature sensitive. This temperature 

sensitivity could be greatly reduced by careful material selection, and 

hysteresis was not a serious problem because changes in the pressure 

measured were always in the same direction. On the other hand, the 

mechanical thermometers and hygrometers had more serious deficiencies, 

especially at higher altitudes. Both had slow response, and the thermometers 

suffered from radiation errors. The problem of sluggish response was 

especially acute for the hygrometers, making them virtually useless at high 

altitude. Because these problems seemed unsolvable within the constraints of 

the mechanical approach, research on electrical sensors had begun in the 

1920s. 

An electrical sensor was employed in the Blair sonde of 1924 and, later in 

the Duckert sonde, which incorporated a thermometer consisting of a capacitor 

with a temperature-sensitive dielectric material. By the mid-1930s, researchers 

had produced temperature-dependent electrical resistors with a fast response 

time coupled with small size to reduce radiation error; however, not all types of 

existing radiosonde designs could be readily adapted to use this superior sensor. The possibility of an electrical hygrometer had been explored, but not satisfactorily resolved. By the time work began on the NBS-Navy radiosonde 

program in 1936, the deficiencies of mechanical temperature and humidity sensors generally were perceived as significant factors in limiting the ability of 

radiosondes to provide data at high altitudes. The NBS team was very aware of 

the potential superiority of electrical sensors. 

73 
74 SMITHSONIAN STUDIES IN HISTORY AND TECHNOLOGY 

9. The radiosonde designed by Dymond at Kew Observatory was adopted 

by Britain's Meteorological Office in 1940 and designated the Kew Mkl. In 

this sonde, inductances (that were varied by an aneroid barometer, 

goldbeater's skin hygrometer, and bimetallic thermometer) were sequentially 

cycled into the LC circuit of a single audio-frequency subcarrier oscillator, 

which modulated the transmitter in accord with sensor output. The Mkl A was 

introduced in 1943. In this model the flat temperature sensor was altered to a 

cylindrical shape, and circuit improvements were made. In 1945, the Mk2 

appeared, which incorporated parts that could be manufactured by less costly 

methods. In 1952, the Mk2B, featuring improved temperature compensation 

of the RF and audio subcarrier oscillator stages of the transmitter, was 

introduced. This model remained in service for the next 25 years, although the 

ground-based equipment was modernized in 1961 by automating the manual 

method of balancing the sonde's signal against a reference standard. In 1977, 

the Kew Mk3 went into service (and is still in use today). This model employs 

a crystal-controlled transmitter to eliminate drift and contains transistorized 

radio frequency and subcarrier stages mounted on a single printed circuit 

board. The sonde continues to use an aneroid barometer and goldbeater's skin 

hygrometer, but the bimetallic thermometer used in earlier models was 

replaced by a resistance-wire type. The computerized ground-based equipment 

developed for the Mk3 performs pre-flight checks, flight computations, and 

data reduction. 

10. According to Churchill (1948:336) German annual expenditure on 

arms was equivalent to $4,500,000,000; a rate triple that of Britain and France 

combined. 

11. Apart from design specifications, such as the 1936 specifications for 

the NBS-Navy radiosonde, there is little published information on the 

accuracy of various radiosonde designs that appeared in the 1930s and early 

1940s. Presumably the accuracy figures for the European designs were similar 

to those of the NBS-Navy type (i.e., temperature to within one degree over the 

range from plus 40 to minus 75 degrees centigrade; pressure to within one 

millibar from 150 to 1050 millibars; and humidity to within three percent from 

0 to 100 percent relative humidity). There is little published data on the inflight accuracy of production versions of the various radiosonde designs. 

12. The electrical sensor was particularly helpful in the technique of 

audio-frequency modulation that Diamond and his NBS associates decided to 

use. An audio oscillator circuit was employed that produced a frequency 

inversely proportional to a vacuum-tube grid voltage. The grid voltage could 

then be varied by electrical sensors. The variable audio-frequency subcarrier 

then modulated the amplitude of the emitted radio wave. This approach was 

especially advantageous because it provided continuous emission on a 

constant carrier frequency that aided radio direction-finding and because it 

facilitated the use of electrical sensors based upon resistance. The required 

resistive temperature sensor was already available, but, because no suitable 

resistive hygrometer existed, a development program was initiated 

concurrently at NBS for application to the radiosonde project. 
Appendix 

Selected United States Patents Related to Radiosondes 

Patent Invention/Purpose 

2,230,779 Pyrometer potentiometer 

2,277,692 Measuring cloud height and thickness 

2,323,317 Altitude measuring 

2,347,160 Radiometeorography/Transmitting apparatus 

2,347,345 Radiometeorography/Transmitting apparatus 

2,352,578 Radiometeorograph transmitter 

2,354,086 Radiometeorograph/Receiving relay apparatus 

2,355,739 Meteorological apparatus 

2,381,009 Chronometric radiosonde system 

2,396,955 Radiosonde 

2,409,155 Radio transmission N.E. Gibbs apparatus 

2,418,836 Remote recording system 

2,425,537 Calibration of radiosonde 

2,454,320 Hygrometer and switch for radiosondes 

2,467,400 Automatic transmission of data 

2,468,703 Chronometric electronic radiosonde system 

2,500,186 Meterological telemetering system 

2,509,215 Radiosonde (Dept. of War) L. Hillman 

2,547,009 Telemetering system (Dept. of War) 

2,591,600 Radiosonde calibration 

2,613,347 Modulator for radiosonde apparatus 

2,689,342 Pressure operated switching device 

2,701,354 Airborne radiosonde apparatus 

Patentee Filed Awarded 

M.J. Johnson May 1937 Feb 1941 

F.W Dunmore May 1938 Mar 1942 

F.W Dunmore Feb 1940 Jul 1943 

C.F.Wallace Feb 1940 Apr 1944 

C.F.Wallace Dec 1941 Apr 1944 

C.F.Wallace Dec 1941 Jun 1944 

J.R. Mackay Sep 1941 Jul 1944 

I.E. McCabe Oct 1942 Aug 1944 

J.A. Siderman Aug 1944 Aug 1945 

K.O. Lange Sep 1942 Mar 1946 

E.L. Schellens Sep 1942 Oct 1946 

B.K. Hawkes Aug 1943 Apr 1947 

J. Hornbostel Aug 1943 Aug 1947 

J. Hornbostel May 1943 Nov 1948 

PR. Murray Aug 1945 Apr 1949 

CM. Hammel Sep 1946 Apr 1949 

M.Kline Apr 1945 Mar 1950 

L.S.Craig Sep 1944 May 1950 

W.D. Huston, J. Grady Oct 1944 Apr 1951 

C.B. Pear Nov 1948 Apr 1952 

W. Todd Sep 1945 Oct 1952 

PR. Goudy Sep 1950 Sep 1954 

A.E.Bennett Dec 1946 Feb 1955 

75 
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Abbot, C, F. Fowle, and L. Aldrich 

1915. New Evidence on the Intensity of Solar Radiation Outside the Atmosphere. Smithsonian Miscellaneous Collections, 45:1-55. 

Anonymous 

1928. The Radio Industry. New York: A. W. Shaw and Co. 

1961. Handbook of Meteorological Instruments, Part II. 551 pages. London: Her Majesty's Stationary Office. 

Armstrong, E.H. 

1917. Some Recent Developments in Regenerative Circuits. Proceedings 

of the Institute of Radio Engineers, 10(8):244. 

Beelitz, P. 

1954. Radiosonden. 136 pages. Berlin: VEB-Verlag. 

Benade, J., and R. Doan 

1935. Apparatus for Transmitting Cosmic Ray Data from the Stratosphere. 

Physical Review, 47:198. 

Blair, W. 

1937. Some Relationships between Radio and Meteorology. Transcription 

of lecture of 7 April at New York University (in the Blair Papers, 

currently in the custody of W. Blair, Jr., of Monmouth Beach, New 

Jersey). 

Blair, W., and H. Lewis 

1931. Radio Tracking of Meteorological Balloons. Proceedings of the Institute of Radio Engineers, 19:1531 -1560. 

Blake, G. 

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