JaiBureau of Standards

Library, N.W. Bldg

OCT 1 81962

\^&ckniccil v|ete 141

CONTROLLED TEMPERATURE OIL BATHS

FOR SATURATED STANDARD CELLS

PATRICK H. LOWRIE, JR.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

Functions and Activities

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with these standards; the determination of physical constants and properties of materials; the development of methods and instruments for testing materials,- devices, and structures; advisory services to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; and the development of standard practices, codes, and specifications. The work includes basic

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NATIONAL BUREAU OF STANDARDS

technical <viote

141

AUGUST 1962

CONTROLLED TEMPERATURE OIL BATHS

FOR SATURATED STANDARD CELLS

Patrick H. Lowrie, Jr. Boulder Laboratories

Boulder, Colorado

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. - Price 25 cents
Contents

Page

Abstract 1

1. Introduction 2

2. Requirements 3

3. Description 4

3.1 General 4

3.2 Main Tank, Baffle, and Rack 8

3.3 The Heater 12

3.4 The Circulating System 13

3.5 Thermoregulator 16

3.6 Heater Control System 22

3.7 Guard Circuit 26

3.8 Electrical Connections to Cells 27

3.9 Pre-heater and Drain Tanks 28

4. Performance 29

5. Suggestions for Imrpovement 30

6. References 31

li
CONTROLLED TEMPERATURE OIL BATHS FOR SATURATED

STANDARD CELLS

Patrick H. Lowrie, Jr.

ABSTRACT

Two oil baths for the temperature control of saturated standard

cells have been designed and fabricated at the Boulder Laboratories

o o

of the National Bureau of Standards for operation at 28 C and 35 C

o respectively. Short term control to better than ± 0. 001 C with dayto-day variations no greater than 0. 002 C has been achieved with

the use of a mercury -toluene thermoregulator incorporating a temperature anticipating device. The circulating system limits

o temperature gradients in the oil to less than 0. 001 C across any

10 inch section. The baths incorporate pre -heat and drain tanks as well as the main temperature regulated tank to facilitate the insertion

and removal of cells and to minimize oil spillage.
1. Introduction

For over fifty years the basic voltage standard has been the saturated Weston standard cell. This cell consists of a cadmium amalgam electrode and a mercury-mercurous sulfate electrode in a saturated solution of cadmium sulfate and is housed in a sealed glass

H-tube [ l] to [ 8] . Saturated cells are used as voltage standards because the average voltage of a group of these cells has been found to

remain constant to within a few microvolts over many decades.

The cells present a problem, however, in that they are very

sensitive to temperature, movement, and light. At a temperature of o 28 C the emf of a saturated cell will change approximately 50 microvolts per degree C, and if there is a temperature difference between

the two legs of the cell, the emf may vary by as much as 300 microvolts per degree C of temperature difference. If a cell experiences

vibration, the emf will vary provided the vibration is strong enough to

disturb the free mercury or amalgam. If a cell is tipped more than

about 45 it very likely will become unstable and therefore useless as

a standard. If a cell is exposed to daylight or fluorescent light for

an extended period it may be permanently damaged. Exposure to

direct sunlight should definitely be avoided. There is some evidence

that incandescent light is less injurious to cells than is fluorescent

light or daylight [ 8] ,but as no quantitative information of the effect of

light on cells is available at present, the cells are generally protected

from all sources of light.

Recently cells have been marketed which are claimed to be unaffected

by tipping or jarring.
3

Though many baths performing the functions described below have

been fabricated, apparently very little has been published regarding

them since the very early papers [ l] [ 2] . This paper describes two

baths that were designed and built at the Boulder Laboratories for

use at 28 C and 35 C respectively. These embody many of the features of similar baths already in use in the Standard Cell Section,

NBS, Washington. The problems encountered and the techniques

utilized in the design, fabrication, and adjustment of the Boulder

baths are discussed.

2. Requirements

From the discussion above, it can be seen that there are three

primary requirements of any enclosure for saturated standard cells.

These are (1) close temperature control with temperature gradients

reduced to a minimum (to minimize temperature differentials between the legs at any contained cell), (2) stability of the medium (for

oil this means minimum turbulence consistent with small gradients),

and (3) protection from light of the space containing the cells. In

addition to these requirements, the baths should have excellent reliability, adequate capacity, and should be designed for maximum

convenience. The specific requirements of the Boulder Laboratories

were as follows: The baths should be capable of containing up to

twelve groups of cells each, with up to six cells per group. The

cyclic variations in temperature should not exceed 0. 002 C. However, the temperature variation from day to day may be as great as

0. 005 C provided the average temperature does not change from

week to week. Provision should be made for making voltage measurements of the cells and for making temperature measurements of

the oil with minimum disturbance to the cells.
4

Since none of the commercially available baths answered all of

the needs of the National Bureau of Standards, Boulder Laboratories,

it was decided that the baths should be fabricated at the laboratories.

After a study had been made of the design of the oil baths in use at

NBS, Washington, the decision was made to design a somewhat different bath. In addition to the features included in the Washington

2

baths, the Boulder baths include a pre -heating tank and drain tank.

3. Description

3. 1 General

Each bath (figure 1) was designed as a complete unit incorporating

the main oil tank, pre -heating tank, drain tank, and all control and

circulating equipment. The outer wall of the bath and most other

parts are fabricated of sheet stainless steel. The bath rests on a supporting framework of angle iron. The lid (figure 2) consists of

two black, hard anodized aluminum sheets separated by one inch of

polystyrene foam (figure 3). It is designed so that only a small section need be opened when measurements are being made. However,

one entire side of the top of the bath may be exposed when cells are

being transferred to and from the oil. The lid is also easily removable so that the bath may be completely exposed for repair should

the need arise.

2 The author is indebted to David Ramaley and John F. Shafer for

their suggestion that a pre -heater be incorporated in the design.
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FIGURE 3. 35 C BATH COVER WITH ONE SIDE REMOVED

TO SHOW POLYSTYRENE FOAM FILLER
8

3. 2 Main Tank, Baffle, and Rack

The main tank is thirty-six inches in diameter, twenty-five

inches high, and holds approximately ninety -five gallons of clear

mineral oil. It is insulated on the bottom and sides by two or more

inches of crushed cork and on top by the air space between the oil

and the lid as well as by the insulation within the lid. Within the

tank is a cylindrical baffle, as shown in figure 4, twenty-eight inches

in diameter, open at both top and bottom. The baffle serves to control the flow of the oil in such a way as to minimize temperature

gradients. In operation the oil is drawn down through the center of

the bath, passes under the baffle walls, and circulates upward outside the baffle. The area between the baffle and the wall of the tank

is less than that contained within the baffle; as a result, the oil

moves faster with higher turbulence as it moves up outside the baffle.

The speed of the oil is then reduced as it moves down over the cells,

and the turbulence is decreased. Thus both thorough mixing and

low effective turbulence are achieved. Bolted to the top of the baffle

is the rack which holds the baskets containing the cell groups (figure

5). The baskets are hung on the rack in such a way as to place the

center of gravity of the group of cells well below the points of suspension. This reduces the possibility of accidentally tipping the

group. The baskets are provided with handles so that the groups may

be easily handled during immersion in or removal from the oil

(figure 6).
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FIGURE 6. STANDARD CELL BASKET

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12

3. 3 The Heater

The heater element is made up of approximately 50 ohms of

number 22 Nichrome wire. It is wound between two hoops of insulating

material to form an open grid through which the oil passes (figure 7).

This type of construction minimizes the heat capacity of the heater,

allowing it to operate at very nearly the temperature of the oil and

to provide the most efficient heat transfer to the oil. The heater is

placed at the bottom of the baffle so that the oil after passing through

the heater has the maximum possible distance in which to mix before passing over the cells. The heater has current in it continuously; the action of the thermoregulator serves only to control the amount

of current.

,o FIGURE 7. CLOSE-UP OF 35 C BATH DURING CONSTRUCTION
13

3. 4 The Circulating System

During the initial experimentation with stirring systems, it was

found that small, high speed impellors were unsatisfactory in that

they introduced considerable heat into the oil. Since one of the baths

was to operate at an oil temperature only five degrees Celsius

above ambient, uncontrolled sources of heat could not be tolerated.

At the suggestion of Dr. Walter J. Hamer, a larger low speed stirrer

was designed. Hence, the oil is circulated by a four -blade stirrer,

25 inches in diameter, rotating at 55 rpm. A four -blade propeller

was used as a stirrer because of its efficiency advantage over the

two -blade type [ 9] • The shaft extends through the bottom of the tank

to a 20 to 1 gear reductor which is connected to a 1/4 hp motor by

means of a belt. Oil leakage through the shaft bearing is prevented

by means of a mercury seal (figure 8). The motor is shock mounted

on a platform under the control circuits section of the bath. It was

originally connected to the gear reductor by a belt to facilitate

changing propeller speed during the initial experimentation. Since

the belt also serves to help isolate any motor vibration from the cells,

it was retained in the final design. The motor is of the induction type

and has sealed ball bearings. A fan is affixed to the motor shaft

to assist in cooling both the motor and the enclosed area under the

bath. The platform on which the motor rests was designed to be

removable, so that the motor could be mounted on the floor or elsewhere in the event that the vibration was too great in its present

position. This, however, was found to be unnecessary.
14

PROPELLER SHAFT

MERCURY

RADIAL BEARING

RADIAL THRUST BEARING

NEOPRENE O-RING

ROTATING SLEEVE

WHITE MINERAL OIL

TANK BOTTOM

REINFORCING WEB

FIGURE 8. PROPELLER SHAFT AND MERCURY SEAL ASSEMBLY
15

Originally the inside of the baffle wall was smooth, and a gap

of approximately one and one -half inches existed between the wall

and the tip of the propeller blade. It was found that this configuration

allowed the oil to slip past the tips of the blades and thus to recirculate without passing through the heater. This caused a rather

large temperature gradient. The problem was solved by affixing a horizontal ring to the inside of the baffle at the level of the propeller (figure 4). This ring projects about one inch from the baffle

wall and effectively prevents the oil from circulating up between the

tips of the blades and the baffle wall. The efficiency of the propeller

was further increased by the addition of two vertical blades, welded

to the underside of opposing horizontal blades, as shown in figure 7,

to thrust the oil toward the wall of the tank after it is drawn down

through the horizontal blades.

The temperature gradients were determined by establishing the

minimum distance between two points in the oil which, when probed

with a platinum resistance thermometer, exhibited a temperature

difference of 0.001 C. After completing the modifications described

above, this distance was found to exceed 10 inches throughout both

baths.
16

3. 5 Thermoregulator

The temperature of the oil is controlled by a thermoregulator of

the mercury-toluene type similar to that described in [ 1] . In this

type of thermostat the mercury serves the two functions of sealing

the toluene in the regulator and of providing a conducting path between

the electrical contacts. Any expansion of the mercury due to a change

in temperature is so small compared to that occurring in the toluene

that it may be disregarded provided the bulk of the mercury column is

under the oil. The amount of mercury exposed to the air should of

course be minimized, since changes in air temperature could cause

erratic action of the regulator. Toluene was chosen as the expansion

element for a number of reasons. It has a high temperature coefficient of expansion (over six times that of mercury) [ 10] , relatively

large surface tension (though quite small as compared to that of mercury), and acceptably low volatility. The regulator was first constructed of stainless steel tubing (figure 4) with the hope that the advantages

of greater safety and convenience of handling and the higher heat conduction of stainless steel, as compared to pyrex, would not be off-set

by the considerably lower dimensional stability of the steel. It consisted of 1/2 inch o. d. tubing bent in the form of a double, concentric

hoop which was closed at one end by a plug and at the other by a

rather elaborate U-tube system. In operation the double hoop was

filled with toluene, and the U-tube was filled with mercury. A small

glass capillary tube was affixed to the open end of the U-tube, and a platinum wire was inserted in the capillary tube to serve as one

electrical contact. The walls of the regulator formed the other contact,

and the mercury served to connect the two. It was found after an extended period of experimentation, however, that long-term temperature stability could not be achieved with a stainless steel regulator of
17

this type. Despite extreme efforts to insure the absence of leaks and

the dimensional stability of the device, it was not possible to reduce

the change in the mean temperature of the oil to less than a steady

rise of 0. 00025 C per day. As this change resulted in an increase

in temperature of 0. 01 C every forty days, the stainless steel regulator was judged to be impractical for the intended purpose and was

discarded. In its stead a pyrex regulator was fabricated, the design

of which was based on that of the regulators used for the same purpose

at NBS, Washington, and at the present time both oil baths are controlled by pyrex regulators.

MERCURY FILLED

J-TUBE

6

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TOLUENE FILLED

SENSING TUBE

y

CAPILLARY

SECTION

RESERVOIR

PLATINUM

ELECTRODE

(SEALED

IN GLASS)

GROUND BALL

STOP COCK

THERM0REGULAT0R J-TUBE DETAIL

FIGURE 9. PYREX THERMOREGULATOR
18

The pyrex regulator consists of a one -inch o. d. glass tube, bent

into the approximate form of a toroid. Sealed to this sensing tube is a small tube bent in the form of a J (figure 9). In operation, the

sensing tube is filled with toluene, and the J-tube is filled with mercury. As in the stainless steel regulator, the mercury serves both

to contain the toluene and to provide an electrical path between two

electrodes. The upper, open end of the J-tube is drawn down to about

0. 6 mm, i. d. , and it is in this section that the platinum wire is inserted. The small i. d. of this section serves to amplify the changes

in the height of the mercury column resulting from the thermal expansion and contraction of the toluene. At the top of the capillary

section is a bulb which serves as an overflow chamber. The J-tube

also has two other tubes sealed to it somewhat below the capillary

section. One of these contains a platinum electrode sealed in the

glass at the junction between the two tubes. This electrode, the

platinum wire inserted in the capillary tube, and the mercury column

comprise the control switch of the system. The second tube that is

sealed to the J-tube is used for filling the regulator and for adding

or removing mercury when adjusting the control temperature and

contains a stopcock to facilitate opening the tube for these purposes.

The J-tube is enlarged in diameter for a short distance from the

junction between it and the sensing tube. This enlarged space serves

as a reservoir for mercury. Obviously great care must be exercised

to prevent the possibility of air remaining in the sensing tube in

order to insure proper operation of the regulator since the presence

of air manifests itself in two undesirable ways: it causes the action

of the regulator to be erratic rather than periodic, and it causes the

temperature of the oil to vary with changes in atmospheric pressure.

These effects are illustrated in figure 10. Therefore, before the
19

toluene was introduced into the regulator it was boiled for a few

4

minutes to insure that all dissolved air was removed.

When boiling toluene appropriate safety measures must be observed

as toluene is highly flammable.

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FIGURE 10. RELATIONSHIP OF TEMPERATURE AND

ATMOSPHERIC PRESSURE RESULTING FROM

ENTRAPPED AIR IN THERMOREGULATOR
20

When filling the regulator with toluene the following procedure was

carried out. The top of the capillary tube was sealed, and a flexible

tube was connected to the arm containing the stopcock. The other

end of this tube was connected to one opening of a three-way valve.

A second opening was connected to a vacuum pump, and the third was

connected to a flask containing the freshly boiled toluene. A vacuum

was then drawn on the regulator, and, by means of the three-way

valve, the toluene was allowed to fill the regulator without contaminating the vacuum pump. The apparatus used in filling the regulator

is shown in figure 11. Even with the precautions noted above, however, a small bubble remained when the toluene ceased to flow into,

the regulator. This bubble was then carefully removed by the following means. The flask of toluene was connected to the regulator by

a flexible tube. The regulator was then cooled by packing ice and

water around the sensing tube. After it had been allowed to cool for

some time it was carefully overturned to trap the bubble in the mercury reservoir chamber. The regulator was then gently warmed by

holding the sensing tube with bare hands, and the expansion of the

toluene forced the bubble out of the tube. The regulator was then

brought to a temperature approximately one degree higher than the

highest temperature at which it would be used to regulate, and the

J-tube was filled with mercury. The regulator was allowed to cool

slowly, and mercury was continually added to replace that drawn into the reservoir by the contracting toluene. Thus the J-tube was

filled with mercury with much less danger to the regulator than that

existing when an attempt was made to fill the tube while gently tapping

it to jar the mercury into place. Because of the fragility of the

thermoregulator an acrylic plastic cover has been placed over that

part which extends above the oil.
21

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3. 6 Heater Control System

The thermoregulator operates a sensitive relay which in turn

operates a singlepole, double -throw relay connecting one of two

variable transformers to the heater. A schematic diagram is shown

in figure 12. By means of the variable transformers the current in

the heater can be adjusted so that in the high-heat condition it has a value slightly above that needed to maintain the desired temperature

and in the low-heat condition has a value slightly below that needed

to maintain temperature. Overshoot is greatly reduced by use of a

separate heater, hereafter referred to as the anticipator, which consists of 30 ohms of No. 22 Nichrome wire wound around the sensing

5

tube of the thermoregulator. Although the main heater is constantly

supplied with current as explained above, the anticipator operates

on an on-off cycle, i. e. , there is current in it during the high-heat

interval but no current in it during the low-heat interval. The anticipator thus causes a larger change in temperature to occur in the

thermoregulator than in the oil, and the thermoregulator also detects this temperature change much sooner than it would if the anticipator were not used. As a result, the current in the main heater

is reduced sooner than it would be without the operation of the anticipator, and overshoot is minimized. The control circuits are contained in a space below the drain and pre -heater tanks (figure 13).

All relays are of the plug -in type to facilitate quick replacement.

Figure 14 shows a close view of the relay chassis. During the testing of the bath it was noticed that the action of the thermoregulator

caused excessive chattering of the relays. The chattering was

eliminated by the addition of an RC delay circuit connected in the

sensitive relay coil circuit. The relay is now delayed about onefifth second on closing and about one -half second on opening.

5 The use of an anticipating circuit was suggested by a regulating

system designed by T. Deighton [ 1 1] .
23

SENSITIVE a 6VDC RELAY

REGULATOR

HEATER CONTROL CIRCUIT

GUARD

REGULATOR

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HI TEMP LO TEMP LO H SENSITIVE SENSITIVE LATCHING LATCH ING NOTE:

RELAY RELAY RELAY RELAY PREHEATER

CIRCUIT NOT

GUARD CIRCUIT SHOWN.

FIGURE 12. SCHEMATIC DIAGRAM OF CONTROL CIRCUITS
24

FIGURE 13. CONTROL CIRCUITS SECTION
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3. 7 Guard Circuit

As the temperature of standard cells must be kept below about

40 C [ 3] to [ 8] , provision must be made in any cell enclosure to

insure that malfunction of the heater control circuit does not cause

the temperature to exceed this value. In addition, it was felt that

even under emergency conditions, in order to minimize temperature

shock to the cells, the temperature of the baths should not be allowed

to vary more than one degree Celsius-. With these factors in

mind a guard circuit (figure 12) was built into the bath. This circuit

wil] completely disconnect the regulator and heater system and connect the heater through the guard thermoregulator to 120 volts a-c

and associated relays. Thus, if for any reason the temperature of

the bath either increases or decreases by about one degree, the guard

circuit will take over and start regulating to about ±0. 1 C at the

new temperature. In addition to the circuitry in the baths, an emergency power generator has been installed which supplies power to

the standard cell room in the event of failure of normal power. The

guard circuit has been designed to remainin control, once it has taken

over control, until manually disconnected. This was done to insure

a positive indication of large changes in temperature that would otherwise be noticed only by the behavior of the standard cells. The

guard regulator is of the bi -metal type.
Z7

3. 8 Electrical Connections to Cells

In order to minimize thermal electromotive forces, copper-tocopper connections are used in all cell emf circuits. Originally an

attempt was made to eliminate any junctions in the wires from the

cell groups to the potentiometer. However, this proved to be very

complex and inconvenient, and a study was made of the effects of

copper-to-copper connections. The conclusion was reached that

connections of this type do not produce measurable thermal electromotive forces. As a further precaution the positive and negative

leads were twisted together as tightly as the insulation would permit

to minimize the effects of the Thomson emf. The connections to

the cell groups are made by methods appropriate to the several forms

of cell racks. In the most common form the cell leads are brought

out to copper pins at the top of the rack and for this type a special

clamp was designed which is shown in figure 15.

FIGURE 15. CONTACT CLAMP
28

3. 9 Pre -heater and Drain Tanks

A small p re-he ate r tank (figure 5) was incorporated into the design of the 28 C bath for use when adding groups of cells to the bath.

Since the NBS groups are contained in this bath, it was suggested

that the introduction of cold (room temperature) groups into the oil

would be undesirable in that it would reduce its temperature temporarily and also cause temperature gradients. This was found to be

the case. The effects last an hour or two after the new group has

been placed. The inclusion of the pre -heater provides for bringing

o o the group and its basket to 28 C ± 0. 2 C before immersion in the

main tank and, as a result, the emf of the reference group remains

undisturbed by the introduction of new cells. Thus the calibration

of cells is not interrupted. A pre -heater was also incorporated in

o the 35 C bath to provide an effective increase in its capacity. As

cells must be kept at temperature for four to six weeks before they

can be calibrated, it was felt that the capacity of the bath could be

considerably increased by providing a second tank in which the cells

could spend the first two or three weeks of this waiting period. As

the temperature requirements on the pre -heater are much less

stringent than on the main tank, an adjustable, mercury-in-glass

thermoregulator that is commercially available was used in a simple

o on-off circuit. The pre -heater in the 35 bath can contain up to ten

groups of cells and thus effectively increase the capacity of the bath

to 22 groups. The drain tanks (figure 5) were incorporated into the

baths to reduce the possibility of spillage of oil on the floor and on

the operator. The cells may be left in either drain tank for an

extended period as the drain tanks also are protected from light.
29

4. Performance

The 28 C bath has been in operation for over a year, and the 35 C

bath has been in operation for about eight months. At first and immediately following any drastic temperature adjustment (the 3 5 C

bath was adjusted to 28 C for a short period), a slow erratic rise

in temperature was noted as shown in figure 1 6. This evidently was

due to the escape of small bubbles of entrapped air in the mercury

column. After some time, however, this change leveled off after a total rise of about 0. 02 C. At present the cyclic change in temperature is less than ± 0. 001 C. This results in a variation in cell emf

of no greater than 0. 1 microvolt. The day-to-day change in mean

temperature does not exceed 0. 002 C. As can be seen in figure 17,

the temperature is still climbing very slowly - roughly 0. 001 C every

20 days. It has been found that the mercury column of the thermoregulator must be cleaned two or three times a year to remove any

foreign material from the top of the column in the capillary tube. If

this is not done, the platinum wire will become insulated by the material, and the temperature will suddenly rise by as much as 0. 01 C.

Even after this occurs, however, the normal state can be reattained

by cleaning the column and the wire.

As a result of the incorporation of these oil baths into the

standard cell unit at the Boulder Laboratories, fluctuations in cell

emf due to variations in temperature have been reduced to such an

extent that the accuracy with which the volt is maintained at the

Boulder Laboratories is no longer limited by temperature variations.
30

5. Suggestions for Improvement

Although the present design objectives were attained by the presently operating baths, it is believed that the convenience of temperature adjustment and system reliability could be further improved by

the replacement of the mercury-toluene regulator and relay system

with an all solid state electronic system. The primary difficulty

with present electronic systems is the lack of long term stability and,

in the case of vacuum tube systems, poor reliability. The solution

of the stability problem coupled with the reliability associated with

the use of solid state devices could lead to an improvement over the

present system. In the event that such an electronic control is perfected, it will be incorporated in the present baths.

The author is indebted to Mr. Frank D. Weaver for his inspiration and assistance during the design of the baths at the Boulder

Laboratories, and also to Dr. Walter J. Hamer and Miss Catherine

Law of the Electrochemistry Section of NBS Washington for their

help.
31

6. REFERENCES

[ 1] F. A. Wolff and C. E. Waters, Bulletin of the Bureau of Standards,

Vol. 4, No. 1 (1907-08)

[2] G. A. Hulett, American Physical Society, Vol. 32, page 257 (1911)

[ 3] The Eppley Laboratory, Incorporated, Bull. 1 (revised July 1956)

[4] Weston Engineering Note Vol. 1, No. 3 (June 1946)

[ 5] Weston Engineering Note Vol. 3, No. 6 (December 1948)

[6] Daystrom Technical Data No. T-7701-1

[7] G. W. Vinal, Primary batteries, (John Wiley, 1950)

[ 8] G. D. Vincent, The construction and characteristics of standard

cells, IRE Transactions on Instrumentation, Vol. 1-7, Nos. 3 and

4 (December 1958)

[ 9] P. Ballif and L. Dryden, Further measurements of propeller

fan characteristics, Journal of Research NBS 6, No. 3, (March

1931, RP 283)

[10] N. A. Lange, Handbook of Chemistry, (McGraw-Hill, 1961)

[11] T. Deighton, A precision thermostat for the temperature reggulation of a room, Journal of Scientific Instruments, Vol. 13,

pp 298-300, (September 1936)
32

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* U.S. GOVERNMENT PRINTING OFFICE: 1962 O - 657707

U. S. DEPARTMENT OF COMMERCE

Luther H. Hodges, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its major laboratories in Washington, D.C., and

Boulder, Colorado, is suggested in the following listing of the divisions and sections engaged in technical work.

In general, each section carries cut specialized research, development, and engineering in the field indicated by

its title. A brief description of the activities, and of the resultant publications, appears on the inside ofthe

front cover. WASHINGTON, D. C.

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Crystallography.

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Standards. Millimeter-Wave Research.

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Center.
NBS