NBS MONOGRAPH 84

Standard Cells

Their Construction, Maintenance,

And Characteristics

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assuring maxinmm application of the physical and engineering sciences to the advancement of technology

in industry and commerce. Us responsibilities include development and maintenance of the national

standards of measurement, and the provisions of means for making measurements consistent with

tliose standards; determination of physical constants and properties of materials; development of

methods for testing materials, meciianisms, and structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the establishment of standard practices for incorporation in codes and specifications; advisory service to government agencies on scientific and

technical problems; invention and development of devices to serve special needs of the Government;

assistance to industry, business, and consumers in the development and acceptance of commercial

standards and simplified trade practice recommendations; administration of programs in cooperation w ith Lnited States business groups and standards organizations for the development of international

standards of practice: and maintenance of a clearinghouse for the collection and dissemination of

scientific, technical, and engineering information. The scope of the Bureau's activities is suggested

in the following listing of its four Institutes and their organizational units. Institute for Basic Standards. Electricity. Metrology. Heat. Radiation Physics. Mechanics. Applied Mathematics. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.*

Radio Standards Laboratory: Radio Standards Physics; Radio Standards Engineering.** Office

of Standard Reference Data.

Institute for -Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic

Materials. Reactor Radiations. Cryogenics.** Office of Standard Reference Materials.

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Instrumentation. Transport Systems. Office of Technical Services. Office of Weights and Measures. Office of Engineering Standards. Office of Industrial Services.

*NBS Group, Joint Institute for Laboratory Astrophysics at the University of Colorado. **Located at Boulder, Colorado.
UNITED STATES DEPARTMENT OF COMMERCE • Luther H. Hodges, Secretary

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

Standard Cells

Their Construction, Maintenance,

and Characteristics

Walter J. Hamer

National Bureau of Standards Monograph 84

Issued January 15, 1965

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 Price 35 cents
Library of Congress Catalog Card Number: 64-60065
Foreword

Accurate measurement of electromotive force is important in many areas of science and

technology. Physical standards for such measurement are provided by standard cells, which

are long-lived electrochemical systems of highly stable electromotive force. This publication gives the origin and derivation of the unit of electromotive force and outlines

the procedures by which the National Bureau of Standards maintains and disseminates this unit

by means of standard cells. Information is also given on the construction, maintenance, and

characteristics of standard cells as well as a history of their development. Emphasis is placed on the precision and accuracy of electromotive force measurements; the stability of standard cells,

especially those of the National Reference Group; and efforts made to construct standard cells of

high quedity.

A. V. ASTIN, Director
Library of Congress Catalog Card Number: 64-60065
Foreword

Accurate measurement of electromotive force is important in many areas of science and

technology. Physical standards for such measurement are provided by standard cells, which

are long-lived electrochemical systems of highly stable electromotive force. This publication gives the origin and derivation of the unit of electromotive force and outlines

the procedures by which the National Bureau of Standards maintains and disseminates this unit

by means of standard cells. Information is also given on the construction, maintenance, and

characteristics of standard cells as well as a history of their development. Emphasis is placed on the precision and accuracy of electromotive force measurements; the stability of standard cells, especially those of the National Reference Group; and efforts made to construct standard cells of

high quality.

A. V. ASTIN, Director
Contents

Foreword iii

1. Introduction 1 2. The unit of electromotive force 1 2.1. Realization ] 2.2. History 4

2.3. Maintenance 5

2.4. Dissemination 8

2.5. International comparisons 10

3. Early standard cells 11

4. The Clark cell 13

5. The Weston (or cadmium sulfate) cell 15

5.1. General 15

5.2. Preparation and properties of materials 16

5.3. Containers for standard cells 19

5.4. Assembly and mounting of standard cells 20

5.5. Electromotive forces of newly made cells 21

6. Effect of variations in components on the electromotive force of standard cells 22

6.1. Concentration of solution 22

6.2. Acidity of solution 22

6.3. Composition of amalgam 23

6.4. Crystal phases of cadmium sulfate 23

7. Characteristics of standard cells 23

7.1. Emf-temperature coefficient 23

7.2. Emf-temperature hysteresis 26

7.3. Temperature range 27

7.4. Emf-pressure coefficient 27

7.5. Internal resistance 28

7.6. Effect of current 28

7.7. Effect of light 30

7.8. Effect of shock 30

7.9. Effect of vibration 30

8. Life of standard cells 30

9. References 31

10. Appendix 1 33

11. Appendix 2 34

12. Appendix 3 35

13. Appendix 4 35

14. Appendix 5 36

15. Appendix 6 37

16. Appendix 7 37

iv
Standard Cells

Their Construction, Maintenance, and Characteristics

Walter J. Hamer

This Monograph contains information on the construction, maintenance, and characteristics of standard cells. The effects of temperature, pressure, electric current, light, shock, and vibration on

standard ceUs are discussed. A history of the realization and maintenance of the unit of electromotive

force is also included. A record of international comparisons of the unit of electromotive force is pre- sented as well as information on the constancy of the National Reference Group of Standard Cells.

1. Introduction

Standard cells are physical representations of

the unit of electromotive force (emf), serve in the

maintenance of the unit, and are used as standards

with which the emf of other cells and systems and

IR drops are compared. Together with standards

of resistance {R) they are also used in the measure- ment of current, /. When measurements of electric power, P, are made in terms of standards for emf

(£") and resistance, the expression for power,

P = E^IR, shows the necessity of knowing E ac- curately, since a small error in the standard for E

would produce a percentage error twice a» great

in the value for the power, P. Standard cells are electrochemical systems composed of two dissimilar electrodes immersed in an

electrolytic solution. They are not intended to supply electric current and, therefore, are of different design from those electrochemical systems

which are intended for such purpose. Owing to their special use, standard cells are required to meet certain performance criteria and, for precise

measurements, to have certain inherent characteristics. They must be reasonably reproducible,

exhibit good permanency, possess low emf-temperature coefficients, have a low or moderately

low internal resistance, be relatively insensitive

to current drains of low magnitude, and, if possible, have an emf of convenient magnitude. Since

a standard cell is a physical representation of a unit

it is obvious why permanency is of prime importance

in a standard cell. The precision with which the

emf of standard cells is measured, accordingly,

exceeds that normally required for other types of

galvanic cells.

2. The Unit of Electromotive Force

2.1. Realization

The practical unit of emf, the volt, is not an arbi- trary one but Uke the other electrical units is de- rived from the basic mechanical units of length, mass, and time using the principles of electromagnetism with the value of the magnetic constant

(the so-called permeability of free space) taken as

47r/10^ in the rationalized mksa (meter-kilogramsecond-ampere) system of units. ^ It has been

customary, following the first use of the term by

Gauss [9],^ to refer to electrical units based on the

basic units of length, mass, and time as absolute

electrical units. ^ The transition from arbitrary to absolute units began with the work of Gauss [9] in 1833 and of Weber [2] in 1851, who showed that

' This is the Giorgi system [1], which is a part of the Syst^me International d'Unith

(SI), adopted in a resolution, 11th General Conference on Weights and Measures.

Paris, October 1960. Other systems for the basic units of length, mass, and time have been used. These include the millimeter-milligram-second system of Weber [2], the meter-gram-second system recommended by the first committee of the British Association for the Advancement of Science appointed to consider electrical units [3], the quadrant-(eleventh-gram)-second or Q.E.S, system of Maxwell [4], the foot-grain- second system used in England for a time (5], and the cgs (centimeter-gram-second system [6] (see Appendix 1). Heaviside also proposed a rationalized system wherein

the cgs unit of emf was increased by a factor V4Tr (unit of resistance increased by a factor 477 and the unit of current decreased by the factor V47r [7]). In all of these, the Giorgi system excepted, the magnetic constant (the permeability of free space) is taken as unity. Regardless of which system is chosen the practical system remains

unaffected. Additional information on electrical units is given in reference [8].

^ Figures in bracitets indicate the literature references ori page 31.

^ It is unfortunate that the name "absolute" has persisted. It is sometimes wrongly

interpreted to imply that there are no errors involved in the measurements or that per- fection has been attained.

it was possible to measure electrical quantities

in terms of mechanical units. Weber pointed out

the desirability of making the electrical units consistent with those used in other branches of science

and engineering.

The electrical units determined in the cgs electro- magnetic (em) system are of inconvenient size for practical use. For example. Sir William Thomson

(Lord Kelvin) [10] in 1851 showed that the emf of

a Daniell cell was about 1 X 10^ cgs em units. Even

so, a Committee of the British Association for the

Advancement of Science in 1873 [6] recommended

the cgs em system for use both in basic science and

practical engineering. However, the practitioners,

although they agreed with the desirability of re- lating electrical units to mechanical ones, objected

to the use of the cgs em system of units in practice

both because of the magnitudes involved and be- cause they had been using such terms as ohms and

volts for their units. Their views prevailed and in 1881 the International Congress of Electricians

[11] meeting in Paris adopted the cgs em system

of units as the fundamental system and the volt- ohm-ampere system for practical use, with the

practical units being made larger or smaller than

the corresponding cgs em units by an appropriate

power of 10.

1
The factors chosen for emf, resistance, and

current were:

1 volt (practical unit) = 10** cgs electromagnetic

units of emf

1 ohm (practical unit) = 10** cgs electromagnetic

units of resistance

1 ampere (practical unit) = 10"' cgs electromagnetic

unit of current. The factor 10" was chosen for emf since then the

emf of a Daniell cell, widely used at that time as a rough standard of emf, became approximately 1 volt. The factor 10" was chosen for resistance, for

in this way the value of the Siemens mercury col- umn, already used as a resistance standard,

especially on the European continent, became

approximately 1 ohm (actually about 0.94 ohm).

The factor for the ampere was then fixed as 10~' by the requirement of Ohm's law.

The unit of emf, although a most important unit,

is obtained from the ohm and the ampere. To

date, no direct absolute measurement of emf in the em system of units has been found feasible. Instead its value is estabhshed experimentally

in em units through Ohm's law and the measure- ment of the fall of potential produced in a resistance

by a current, each of these being capable of deter- mination in absolute measure.'*

The ohm in absolute measure is usually obtained

in terms of length and time by means of inductance

and frequency. In the Wenner [15] method, a mutual inductor of known dimensions, and thus of

calculable inductance, is placed in a suitable cir- cuit containing the resistor the value of which is to be determined, a battery, a galvanometer, and two

rotary reversing switches. The primary of the

mutual inductor is placed in series with the battery

and resistor, and with one rotary reversing switch

so arranged as to reverse the cormections to the

primary. The secondary of the mutual inductor

is connected to the potential terminals of the re- sistor through a galvanometer and a rotary revers- ing switch for reversing the connections to the

secondary terminals. The galvanometer detects

the balance between the induced emf in the sec- ondary and the hR drop across the resistor. When

balance is obtained, i.e., when the galvanometer

shows no deflection, IrR = '^nIpM where h is the current in the resistor, I,, the current in the primary,

M is the computable mutual inductance, and n is

* The unit of emf may also be determined in cgs electrostatic {es) units, with the

elective constant (the so-called permitivity of free space) taken as unity, using absolute

electrometers. The accuracy of the methods, however, is much lower than the em

approach, the order of magnitude being 1 part in 10,000. The unit of emf in es units

is. therefore, arrived at indirectly by converting em values to es values using the ex- perimental factor, 12.997925 ± 0.000Q0.3)x 10'" cm sec"', the speed of propagation of an electrical disturbance (within experimental error, the speed of light in free space). As an example. Sir William Thomson (Lord Kelvin) (12] found the emf of a Daniell

cell to be 0.(X)374 cgs es unit from the attraction between two parallel disks ctinnected

to the opposite poles of a Daniell cell. This value of Thomson is equivalent lo 1.121 X 10" cgs em units or 1.121 volts. An atomic standard for the volt, although most desirable, has not been forthcoming

to date. The Stark effect has been proposed [131 I*"' can be used only to measure

relative v<»llage and is of insufficient accuracy, with the limiting factttr residing in the measurement of the electric dipole moment [14).

the frequency of commutation in cycles per second

{2 n — a. number of reversals per second of the rotary

reversing switch in the secondary). The circuit is so arranged that during that portion of the cycle

when the primary of the mutual inductor is in series

with the resistor, I, — I,/, thus

/? = 4raM. (1) Since M is calculated from dimensions with refer- ence to length standards and from the permeability

of the medium, and n is measured indirectly in terms of the unit of time, R is given in terms of the

basic units of length and time and of the permeability of the space surrounding the windings of the

inductor. The overall precision (repeatability) of

the method is ±5 parts per million (ppm). For more details, reference [16] should be consulted.

The ohm in absolute measure may also be obtained

with similar accuracy using a self inductor [17], or with lesser accuracy by measuring the relative rotation of a coil and a magnet or the motion of a

coil in the earth's magnetic field. Although the

latter methods are now obsolete they are mentioned

here because of their historical importance in the

evolution of the electrical units. The ohm in absolute measure may also be ob- tained using a computable capacitor [18]. In this method the evaluation of resistance is based on a nominally 1-pf capacitor whose value in es units

may be calculated with high accuracy from its di- mensions and thence in em units using the speed

of Ught. This capacitor is then used to calibrate

0.01-/xf capacitors, the admittances of which are then compared with that of a 10^-ohm shielded a-c resistor using a special bridge network. A con- ventional d-c step-down is then used to provide

"absolute" calibration of d-c resistors of lower

magnitude, specifically 1 ohm. For more details,

reference [18] should be consulted. The overall precision (repeatability) of the method is about ±2

ppm. Operationally, this method is less involved

than the inductance methods and may be used on

an annual basis to check on the constancy- of the

resistance standard.

The ampere in absolute measure is obtained in terms of length, mass, and time with a current balance [19]. In the current balance the electro- dynamic force of attraction or repulsion, in the x

(vertical) direction, between two coils (one movable

and one stationary) through which a current is flowing is balanced against the force of gravity,

mg, acting on a known mass, m, at a location where

g is the acceleration due to gravity. From this force and the measured dimensions of the coils (from which the rate of increase, dMjdx, of the mutual inductance between the stationary and movable coils may be calculated) together with the

permeability of space in which the coils act, the current is expressed in terms of units of length, mass, and time, where time enters into the calculations through the acceleration of gravity. The

overall precision (repeatability) of this method is about ±6 ppm. The ampere may also be obtained

2
with an uncertainty of about ± 8 ppm by an electro- dynamometer in which a torque rather than a force

is measured [20]. The combined precision (re- peatabihty) in determining the ampere in absolute measure is about ±5 ppm. For more details references [19] and [20] should be consulted.

The ampere in absolute measure may be obtained

qualitatively by using a tangent or sine galvanometer

where the current is measured in terms of the deflection of a magnetic needle, the radius of the

galvanometer coil, and the horizontal component

of intensity of the earth's magnetic field. Although

these measurements are now obsolete they are of

historical importance; see later. The current balance methods are involved and accordingly are used only about once every five or ten years. However, in recent years, following the discovery of

nuclear resonance, a new method, although not an

"absolute" one, has been developed to check on the

constancy of the ampere as maintained at NBS in terms of the physical standards of resistance and

emf. This method [21] is based on the determination of the gyromagnetic ratio of the proton in which the precession frequency of the proton is measured in a magnetic field known in terms of

the "absolute" ampere and the constant of an ac- curately made solenoid. By this method drifts in the national ampere of about ± 1 ppm over a period of years can now be detected; also the

method may be used annually to check the con- stancy of the ratio of the national standards of emf

and resistance (see sec. 2.3). For more details on

this method, reference [21] should be consulted.

The volt in absolute measure is obtained at the same time a measurement of the absolute ampere

is made. The usual procedure is to connect in series with the coils of the current balance a re- sistor having a value, R, previously obtained in absolute measure. The voltage drop, IR, in this resistor is opposed to the emf, E, of a standard

cell. When the current, /, is held at such a value, that IR balances E and at the same time the mass, m, of the weight has been adjusted to balance the

electrodynamic force, the relation

E = R[mgl(dMldx)yi^ = RI (2) follows and gives the emf of the standard cell in absolute measure. The experimental uncertainty

in the balance between the emf of the standard

cell and the IR drop in the standard resistor is less than 0.1 microvolt. Considering the experimental

precisions of the absolute determinations of R and /, the precision in the determination of the volt in absolute measure is about ± 7 ppm (rms).

The volt, as now maintained in the United States,

may exceed the absolute volt by as much as 18 ppm

or as httle as 2 ppm. Driscoll and Cutkosky [19]

in 1958, as a result of measurements with a current balance and a PeUat electrodynamometer reported

that

1 NBS ampere = 1.000010 ±0.000005 absolute

amperes.

In these measurements they used a value for gravity 17 ppm lower than that derived from Potsdam.

Recent studies have shown that — 13 ppm is a better

correction of the value derived from Potsdam.

Using this correction,

1 NBS ampere = 1.000012 ± 0.000005 absolute

amperes.

Cutkosky [18] in 1961, as a result of studies with

precision computable capacitors, found that ^

1 NBS ohm = 1.0000006 ±0.0000021 absolute ohms.

He also, by recalculating the results of Thomas,

Peterson, Cooter, and Kotter [16] obtained with a Wenner mutual inductor, gave

1 NBS ohm = 0.999997 absolute ohm.

(J. L. Thomas has estimated the uncertainty in this value to be ±0.000005). From these relations for the ohm and using the second relation for the

ampere,

1 NBS voh = 1.000013 ±0.000005 absolute volts,

or

1 NBS voh = 1.000009 ±0.000007 absolute vohs.

The NBS volt, as now maintained by standard

cells at the National Bureau of Standards, is the one disseminated. When additional data on the

above relations are obtained in the United States and other countries, the International Committee

of the Bureau International des Poids et Mesures,

Sevres, France, which, by international treaty, has

authority to coordinate the standards of measure- ments in the field of electricity as well as of length

and mass, may recommend adjustment in the electri- cal units. Until then, the units as maintained and

disseminated by NBS serve to place laboratories

in the United States on the same standard basis,

accepted by all nations cooperating in the Treaty

of the Meter. The emf of any or all standard cells could be

determined in like fashion. Obviously, such a procedure would be cumbersome and time con- suming and if required would be unfortunate,

indeed. Absolute measurements are involved,

require painstaking work and, therefore, are unsuitable for frequent or routine measurements of

emf. To circumvent the necessity for frequent

absolute measurements, standard cells ^ are constructed, their emfs are determined in relation to current and resistance in absolute measure, and

these cells are then used to maintain the volt; these

This value differs by 1.7 ppm from ihat (I.OOOOO23) published by Cutliosky [18). He has since uncovered an error of this magnitude in the pubHshed value.

* Standard resistors, usually hermetically sealed coils of annealed manganin wire

[22], are used similarly to maintain the unit of resistance. Owing to tlie transitory nature of an electric current no physical standards for an ampere have been, to date,

possible. Instead the standard ampere is given by the ratio of the values for the stand- ard ceil and (he standard resistor.

3
cells are also used to assign emfs to other cells that may be constructed. In other words, the results of

absolute measurements are preserved in a physical

object, the standard cell.*' The vahdity and reali- zation of this approach depends on the possibility

of constructing standard cells, the emfs of which

are independent (or nearly so) of time; otherwise,

the unit of emf would be lost or drift in value in the

interim and a repetition of absolute measurements

would be imperative at frequent intervals. It is obvious that this matter was and is of critical importance. Over the years extensive work has

been conducted to find that electrochemical system

which would exhibit constant emfs for long periods

of time, say decades. Fortunately, such an elec- trochemical system (see below under the Weston

Cell) has been found. The manner in which the

unit of emf is maintained by a group of standard

cells is discussed below under maintenance.

2.2. History

In 1893 the International Electrical Congress

meeting in Chicago chose the Clark cell, a cell devised by Latimer Clark in 1872 [23], as the standard of emf to which they assigned a value of 1.434

international volts at 15 °C in terms of the then

accepted standards for the ohm and the ampere.

The Clark cell and this value for it were legalized as the standard of emf in the United States by an

act of Congress, July 12, 1894 (see Appendix 1). The value, 1.434 international volts at 15 °C, fol- lowed from the determinations made by Kayleigh

and Sidgwick [24], Carhart [25], Kahle [26], and

Glazebrook and Skinner [27] who used current bal- ances or silver coulometers (the electrochemical

equivalent of silver having been determined by

absolute methods) to determine current and B.A.

coils ^ or Siemens mercury columns (or British

' B.A. coil refers to British Association for Advancement of Science coil of silver- platinum alloy (66.6 percent Ag, 33.4 percent Pt); the B.A. ohm = 0.9866 international ohm. The Siemens ohm standard consisted of a column of chemically pure mercury

1 meter long and 1 square millimeter in cross-section at a temperature of 0 "C. Carhart and Kahle used Siemens units and then converted their results to British legal ohms. The British legal ohm was represented by B.A. coils, the resistances of which were

related to the 1884 "legal ohm," the resistance at 0 *C of a column of mercury 106 centimeters long and one square millimeter in cross-section; one British legal ohm =0.9976 international ohm; one British legal ohm = 1.06 Siemens ohm. Table 1. Absolute measurements of the electromotive force of the Clark cell

Experimenters Temperature Resistance unit Current method

Experi- menters'

value (prior

to reduction)

International values

at observed

temperature

at 15 X

Celsius volts volts volts Rayleigh and Sidgwick 15.0 B.A. ohm current" balance 1.45'39 1.4344 1.4344 Carhart 18.0^ Siemens ohm [ silver coulometer 1.434^" 1.431 1.434 Kahle 15.0 Siemens ohm silver coulometer'* 1.4377 ^-f 1.4342 1.4342 Glazebrook and Skinner 15.0 B.A. ohm* silver coulometer 1.4537 1.4342 1.4342

average 1.4342 rounded

value 1.434

" - Carhart [28] later stated that the value 1.434 V was for a mean of two experiments, one at 18 "C and one at 17 °C; a correction for this does not alter the rounded value of 1 .434 V

given above for the absolute emf of the Clark cell.

1 B.A, phm= 0.9866 international ohm. — Carhart and Kahle converted their values from Siemens units to British legaT units, using the relation 1 British legal ohm = 1.06 Siemens ohm. — 1.118 mg silver deposited per second per ampere (international value of electrochemical equivalent of silver). '-B.A. volt. ^—British legal volt.

1 British legal ohm = 0.9976 international ohm.

legal ohms) ^ (known in cgs em units) as standards

of resistance. Their results are summarized in table 1. In each case, the results were converted

to a common international basis (see Appendix 1 for the defined international units). The emftemperature formula of Lord Rayleigh [24, 29] which was then available was used to convert all the values

to a temperature of 15 °C. The values were re- ferred to as "iniernational" because international

agreement had been attained; it was also realized

at the time that additional work would be needed to place the values on a basis truly representative of

the theoretical cgs em units. In the years immediately following 1893 most

countries adopted 1.434 V for the emf of the

Clark cell at 15 °C. Later work showed that cells made with specially purified mercurous sulfate (see

below for a description of the Clark cell) had an emf 0.0003 V lower than 1.434 V obtained for cells made prior to the meeting of the Chicago Inter- national Electrical Congress. Germany, however,

in 1898 recommended and adopted 1.4328 V at 15 °C for the Clark cell based on the German values

for the ohm and ampere. Although the German

value was not universally accepted, it was a more

nearly correct value as later experiments showed.

Also during the years 1893 to 1905 the standard ceU

devised by Edward Weston [30] was found to have

many advantages over the Clark cell and at an

informal international conference called by the

Physikalisch-Technische Reichsanstalt at Charlottenburg in October 1905, the Weston TVormal Cell

(see later for description) was first proposed as a standard to be used for maintaining the volt and was officially adopted in 1908 at the London Inter- national Conference on Electrical Units and

Standards [31]. This London Conference went further and

adopted provisionally 1.0184 V as the emf of the

Weston Normal CeU at 20 °C and recommended for the emf-temperature coefficient of the Weston

Normal CeU the formula based on the measurements

of Wolff [32]. The London Conference stiU felt, however, that further work was needed and recom- mended that additional experiments be made.

4
Accordingly, scientists from England, France, and

Germany met with United States scientists at the

National Bureau of Standards in 1910, and as a

result of their experiments with a large number of

Weston normal cells and silver-coulometer determinations adopted 1.0183 V as the emf of the Weston

Normal Cell at 20 °C. Values derived from this were later assumed to be significant to the fifth, sixth, or seventh decimal as a basis of measurement

[33, 34]. This value (1.0183 V) was still characterized in "international" units since the basis on which the new measurements (1910) were made was

the same as it was in 1893; the precision of the

measurements was higher, however.

Values assigned to the reference groups of each

cooperating nation served as the fundamental basis

of aU emf measurements from 1911 to 1948. By

1948, after interruptions caused by the two World

Wars and after improvements in techniques, an

accurate determination of the electrical quantities

in cgs em units was achieved, and on January 1, 1948, changes from international to absolute units were officially made internationally.^ The legal status of these new units in the United States is exactly the same as that of the older ones because

the law of 1 894 (see Appendix 1) mentions both sets

* Although "international" and "absolute" are frequently used in referring to the

units it is now best to consider these terms of historical interest only. There is only one kind of volt in the em system of units, and the above terms had significance only during the period during which efforts were being made to achieve the theoretical unit.

It is deemed advisable to speak, therefore, of only one kind of volt; when the term

"volt" is used, "absolute volt" is implied, i.e., the volt has a direct relation to the

theoretical unit.

of units on an equivalent basis. However, in order

to remove the ambiguities of the old act, new

legislation was passed by the Congress in 1950 (see

Appendix 2). The changes [35] for the United

States were:

1 international volt (US) = 1.000330 absolute volts

1 international ohm (US) = 1.000495 absolute ohms

1 international ampere (US) = 0.999835 absolute ampere

1 international coulomb (US) =0.999835 absolute coulomb

1 international henry (US) = 1.000495 absolute henries

1 international farad (US) = 0.999505 absolute farad

1 international watt (US) = 1,000165 absolute watts

1 international joule (US) = 1.000165 absolute joules

The conversion factors in other countries were

nearly the same as these. The emf of the Weston

Normal Cell at 20 °C on the new basis then became

1.01864 V. This value is now of historical interest

only.

The relation of the fundamental units to the

measurement of power and energy in both the new

and former systems of units is shown diagrammaticaUy in figure 1. The left half represents the fundamental units and standards maintained by the

National Bureau of Standards, the right half the

units and standards used by the pubhc.

2.3. Maintenance

The unit of electromotive force in the United

States was originally maintained (1897-1906) by

seven Clark cells, the mean emf of which was as- signed a value of 1.4337 V (this value was 0.0003 V

lower than the value recommended in 1893 by the

INT. OHM

Mercury Ohrr

INT, OHM

Wire Stondard

INT OHM

Secondory Sid

AMPERE

INT AMPERE INT. VOLT INT. VOLT

'Silver Ampere* Soturoted Cell Unsoturated Cell

Time r

INT JOULE

(Energy)

THE FORMER INTERNATIONAL ELECTRICAL UNITS OF 1911 TO 1947

OHM

OMM OH M

Wire Standard Secondory Std,

VOLT

Soturoted Cell VOLTUnsoturated Cell

THE PRESENT ABSOLUTE ELECTRICAL UNITS IN USE SINCE JANUARY 1st 1946

Figure 1. Diagram showing the relation of the present and former systems of electrical units to the basic mechanical units of length, mass, and time.

744-338 0-65—

5
Chicago International Electrical Congress because

specially purified mercurous sulfate had been used

in the preparation of the cells; it was 0.0009 V

higher, however, than the value then recommended

by Germany). In 1906 the standard consisted of

both Clark and Weston Normal cells and after 1908

of Weston cells only; an emf of 1.019126 V was as- signed to the mean of the Weston cells at 20 °C

based on a direct comparison with the Clark cells (on the German basis the emf of the Weston

cells was 1.018226 V which was only shghtly lower

than 1.0183 V found in 1910 by the International

Committee which met at the National Bureau of

Standards). After 1911, this latter unit was maintained in each country until 1948 when "absolute"

units were adopted. The number of cells constituting the national standard has varied from time

to time; when a cell shows a steady change from its previously steady value it is removed from the

group (see later for criterion and procedure).

Today, the National Reference Group of Standard

cells consists of 44 saturated Weston (or cadmium

sulfate) cells, all of which have been made at the

National Bureau of Standards from highly purified

materials and assembled under controlled conditions (see later for details). The National Standard

is based on the mean emf of these 44 cells. The

emf of any one cell in the group is equal to the mean

emf, less the average deviation in emf of all 44

cells from the emf of a selected reference cell in the group, plus the deviation in emf of the individual cell from that of the selected reference cell,

Ec (in volts) ^Em-^+ iEc- Er) (3) where Ec = the emf of an individual cell, Em = the mean emf of the 44 cells, 2A = the algebraic summation of the differences between the emf of a se- lected reference cell, Er, and the emfs of all of the

other cells in the group. This reference group (of

cells) consists of three parts, of 11 "neutral" cells made in 1906, 7 "acid" (0.05N) ceUs made in 1933,

and 26 "acid" (0.026N) cells made in 1948. The

last two groups of cells were added to the reference

group in 1937 and 1955, respectively. The meaning

of the terms "neutral" and "acid" appears later. This National Reference Group of cells is also sup- plemented by a group of cadmium sulfate cells made with 98 percent deuterium oxide (heavy

water) [36], the emf of which is about 380 /laV lower

than that of cadmium sulfate cells made with nor- mal water. These ceUs are discussed in Appendix

3, but suffice it to say here that a study of the ratio of the emfs of cells made with normal and heavy

water offers an auxiliary check on the stabihty of

the national unit of emf.

Obviously, all of a group of "identical" cells may increase or decrease in emf with time without

departures from the original assigned mean becoming evident. Therefore, an alternative type of

standard cell of approximately the same emf as

' Measuremenls of large emf differences of ihe order of magnitude of 0.3 V to 0.5 V

with the precision required imposes a problem, though not an insurmountable one.

the Weston cell ^ but of different composition

would be most valuable, for if changes in emf with

time in two different systems occurred, they would

not be hkely to follow the same pattern. Thus,

studies of the ratio of emfs of two different systems

over a period of years would give valuable insight

into the stability of the standard. It is for this rea- son that the National Standard or Reference Group

was eventually designed to include "neutral" and

"acid" cells and to be supplemented by cadmium

sulfate ceUs made with heavy water. The ratio of

or the difference between the emfs of "neutral" and

"acid" ceUs is foUowed in the course of maintaining

the unit of emf.

A cell is removed from the reference group when

its emf has drifted by more than 1.0 /xV from its previously steady value. When a ceU is removed,

the mean emf of the group is "recaptured" by

reverting in the records to the time the cell had been

added to the reference group, calculating a new mean for the reduced group (less the cell removed)

at that time, and finaUy carrying the new mean

forward. In some cases it has entailed going back

as much as 20 years. Since the cells have closely

agreeing emfs the removal of one cell has only a minor effect on the mean emf of the group; this effect has generaUy been below 1 /iV. The effect on the emf of the mean of removing one ceU obviously

is smaller the larger is the number of ceUs in the

group (for practical reasons there is a Umit to this number; if too high a number, it might not be

possible to measure the emfs of aU the cells within

any one day — this would then increase the problems

associated with maintenance).

It is difficult to provide incontrovertible evidence

regarding the long-term stabiUty of the volt maintained with saturated standard ceUs. A considerable body of evidence indicates, however, that it is very unlikely that the unit of emf preserved with the

National Reference Group of Standard CeUs has

changed by any significant amount in the last 53

years. This evidence foUows:

(1) In terms of measurements with silver coulometers (or voltameters) and standard resistors — prior

to 1948 silver coulometers were used in defining the

international ampere (see Appendix 1). To date,

however, there are no international specifications

that enable the coulometer to be used unambiguously as a means of reproducing the international

ampere. Even so, experiments repeated with a

given type of coulometer under the same conditions

after a lapse of a number of years can serve to establish the same current to high accuracy. From

1910 to 1912, inclusive, [37] a series of experiments were made at the National Bureau of Standards with

a Smith form of coulometer and the results were

expressed in terms of the emf of the saturated

Weston ceU at 20 °C; the average of 55 experiments

gave 1.018274 V for the Weston ceU at 20 °C. In

'° Cells that differ much more in their chemical composition than "neutral" and

"acid" cells or "normal water" and "heavy water" cadmium sulfate cells would be even more desirable for this purpose. However, to date, the prime requirement of constancy in emf has not been as well realized in other systems as it has been in cells of the cadmium sulfate type.

6
1931 a series of coulometer experiments were con- ducted at the Physikalisch-Technische Reichsanstalt by representatives from Germany, Great

Britain, and the United States [34, 38, 39]; the

results obtained by the United States with the Smith

form of coulometer gave 1.018273 V for the Weston

cell at 20 °C." Assuming that the unit of resistance

had not changed during this interval of time the

results indicate that the drift in the unit of emf was

not over 0.05 fxV per year.

(2) In terms of measurements with current balances

and standard resistors — in 1934 Curtis and Curtis

[40] using a current balance originally used by Rosa,

Dorsey, and Miller [41] found that 1 NBS international ampere (given by ratio of NBS units of emf

and resistance) was equal to 0.999928 absolute

ampere, whereas the results of Rosa et al. in 1911

gave 0.999926 absolute ampere. Accordingly,

assuming that the unit of resistance had not changed,

these results indicate that any apparent drift in the

unit of emf could not have exceeded 0.1 /xV per

year. Also, since 1938 all checks of the NBS unit

of emf have agreed within the precision (± 7 ppm)

of absolute measurements made with current balances or electrodynamometers and self or mutual inductors; these checks indicate that the

unit of emf is constant to ±0.2 to ±0.3 fiY per year

if the unit of resistance has remained unchanged.

(3) In terms of the gyromagnetic ratio of the proton —in these experiments the precession

frequency, as stated above, is measured in a magnetic field known in terms of the absolute

ampere and the constant of an accurately made

solenoid. The solenoid current, when compared

with that given by the ratio of the NBS standards of

emf and resistance, has been found to have the same value to better than 0.1 /u, A during the interval

from January 1960 to March 1963.^2 If the unit of

resistance remained constant during this period,

the unit of emf has also remained constant to better

than 0.1 /LiV per year. The construction and properties of standard resistors and standard cells are

radically different; the possibility that the two

systems should drift in such a manner as to maintain

a constant ratio seems remote. The accuracy of

absolute resistance measurements has increased

significantly in recent times. Modern absolute

resistance measurements coupled with reference to atomic constants should, in the foreseeable future,

supply convincing evidence regarding the stability of both resistors and cells.

(4) In terms of "neutral" and "acid" cells — the

difference between the average emfs of the "neutral" and "acid" cells in the National Reference

Group of Standard Cells has increased by 8.5 fxV in 26 years, with the increase being only 1.0 ^tV during

the last 10 years [42]. This comparison was made

for 7 "neutral" and 11 "acid" cells, or for cells of

" These results published by Vinal [34] were reported to the fifth decimal and

Vinal said 'The value for Washington in terms of B.S. unit is, however, exactly the same as the result of the bureau's own experiments on the voltameter [37] published

in 1913" (actually 1914). These data have been recalculated, retaining the sixth decimal, and are given in Appendix 4. " The author is indebted to Forest K. Harris and Raymond L. DriscoU for this information.

this type in the National Reference Group since

1937. Since the emfs of "neutral" and "acid" cells have remained relatively constant during the last 10 years the earlier drift in their difference may be

attributed to an aging effect exhibited by the "acid"

cells (the "neutral" cells were 26 years old at the

start of the comparison between the "neutral" and

"acid" cells). For the last 10 years the average an- nual change has been 0.10 fxY.

(5) In terms of international comparisons —in 1948

the units of emf of the United States (USA) and the

Bureau International des Poids et Mesures (BIPM)

agreed whereas in 1960 the USA unit was 1.9 /iV

smaller than the BIPM unit. Assuming that the

BIPM unit had remained constant during this time

the USA unit has changed at a rate of —0.16 )U.V per year. Assuming that the BIPM and USA units

changed at the same rates but in opposite directions

the USA unit then changed at a rate of —0.08 /aV

per year. Additional data on international comparisons are given in section 2.5.

(6) In terms of customers' ce//s — customers'

saturated cells, checked in terms of the National

Standard of emf, show, on the average, an annual

variation of ± 1.2 /u.V per year, but no drifts in emf in one direction or the other. This relative stability refers only to cells that are about 3 or more years of

age; saturated standard cells usuaUy exhibit an

aging effect of several microvolts during the first 3

years after their construction.

(7) In terms of newly prepared ce//5 — saturated

standard cells freshly made from new materials

usually agree with old or aged cells within 5 ^tV. Of course, in this type of comparison the cells must

be made with amalgams of the same percentage of

cadmium and with cadmium sulfate solutions of

the same acidity with respect to sulfuric acid. The

point here is that saturated cadmium cells, made at different times to essentially the same specification, are highly reproducible. Cells made in the

past cannot have drifted seriously in emf if similar

cells of recent construction agree closely in emf with

them.

(8) In terms of the relative emfs of cells within a

group of cells — the differences in emf between the

cells in the National Reference Group of Standard

Cells have remained remarkably constant for decades. Although, as stated above, all "identical"

cells may increase or decrease in emf without evi- dent departures from their mean emf, constancy

in the difference between the emf of individual cells in a group of cells with time nevertheless, in combination with items (1), (2), and (3) above, increases

confidence in the constancy of the mean emf of a group of cells. Although the above remarks apply to the stability

of standard cells in the National Reference Group

of Standard Cells they should also apply to any

standard cell of the saturated type providing it is a quality cell properly maintained. Cells made with

impure materials or poorly assembled wiU invaria- bly show much less stability in emf. In section 2.5 the need for adjustments in the assigned values to

7
standard cells, although infrequent, is discussed.

The need for these adjustments is not clear. It may be that the cells were of poor quality, were

affected by transport, or had not come to equilibrium

after temperature changes which are involved in international comparisons.

The standard cells of the National Reference

Group are housed in slowly stirred oil baths maintained at 28 °C under diffuse light in an air-conditioned room maintained at 25 °C ± 1 °C; the relative humidity automaticaDy remains below 50 percent.

The temperature of the baths is maintained at 28 °C

within 0.01 °C on a long-term basis and within 0.001

° C during measurements using a Gouy controller [43]. In a Gouy controller, a steel piano wire extending

into the mercury of the mercury-toluene regulator is connected to a wheel which revolves at a slow rate whereby the wire is made to periodically make and

break contact with the mercury. The design of

the bath and its temperature control are described

in Appendix 5. A special mineral oil, having the

characteristics listed in Appendix 6, is used as

fluid in the constant-temperature baths. Tests made to determine the temperature of the oil at various locations within the baths indicated a uni- form temperature within 0.001 °C, i.e., no hot or cold spots prevail within the bath or in the vicinity

of the standard cells. This design follows closely

that described many years ago by Wolff and

Waters [44]. A bath for use with saturated standard cells has also been described recently by P. H.

Lowrie, Jr. [45]. The cells are supported about 2 in. below the

surface of the oil on seasoned mahogany strips

about 16 mm wide (slightly smaller than the distance

between the limbs of NBS H-shaped ceUs) and about

10 mm thick and either 24 or 48 cm long, with

grooves into which the cross-arms of the cells fit snugly. Some racks carry 18 cells equally spaced,

except for a somewhat wider space at the middle of

the rack where the rack is supported; others carry 9 cells. Hard rubber, bakelite, or Incite strips

Figure 2. Oil baths used to house the National Reference Group of Standard Cells.

(widened U-shaped), about 40 mm long, 10 mm wide,

and 10 mm thick in the center and 13 mm at each

end, are mounted between cells across the underedge of the mahogany strip. In the top of each

strip, at each end, is inserted a short copper rod

provided with a pair of holes, one 1 mm and the

other 2 mm in diameter and both about 9 mm in depth; these holes serve as mercury cups, one for a

cell terminal and the other for external connections.

These two copper cups are spaced at the same

distance apart on each strip, so that any cell can be

put in the electric circuit by a stabber consisting of a pair of stiff copper wires mounted in a Incite block

(copper wires are amalgamated on the tips). This

is a copper-copper connection through the mercury

and under the oil. From this point on all contacts

are copper-copper to avoid thermal emfs. The

other end of the stabber connection goes to a post

position. There are two post positions, one for the

Reference Cell and one for the Unknown Cell. The leads of the cells go from the posts through

conduits to another post at the emf-measuring

instrument (see later), where the two cells are placed in series opposition by joining the negatives

and the difference is measured.

A photograph showing the oil baths that house

the National Reference Group of CeUs is shown in figure 2. Other baths of intermediate size are also available for housing cells on test. The temperature of the baths is measured with a platinumresistance thermometer and a Mueller bridge,

having a sensitivity of 0.0001 ohm, corresponding

to 0.001 °C. 2.4. Dissemination

Comparisons of the emf of standard cells, such

as may be required in the dissemination of the unit

of emf, i.e., the transfer of the unit from a standardizing laboratory, must or should be carried out by

procedures of the highest precision involving a minimum of uncertainty. In this case, comparisons

are made between standard cells, the emf of which

have been determined in absolute units and other

standard cells, the emf of which have not been so determined. The former may be called reference

cells and the latter unknown cells. These inter- comparisons may be made with high accuracy with

potentiometers by the opposition method (preferably called the difference method). In this method

the two cells (reference and unknown) are connected

in series with their emfs in opposition and the

difference in emf between the two is measured

with a potentiometer using a galvanometer of high

sensitivity. Because the difference in emf between

the two cells is small, only a moderate percentage

accuracy in its determination is required to give the

emf of the unknown cell accurately in terms of the

known reference cell. If the difference between

•the two cells were 100 /u,V the accuracy of the

measurements need be only 1 percent to give the

difference to 1 /aV or only 0.1 percent to give an accuracy of 0.1 ^tV. This method is not direct reading, however, and

8
the operator must determine which of the two cells has the higher emf. The greatest source of error

in measurements by the difference method is the

presence in the circuit of spurious emfs, such as those from thermoelectric action. At the National

Bureau of Standards a special comparator, designed

by Brooks [46], is used. This comparator is direct

reading and compensates for parasitic thermal emfs.

The reference and unknown cells are connected in series-opposition in series with a galvanometer and

an auxiliary source of a few microvolts and adjustments are made in the auxiliary source until the

galvanometer indicates a null balance. The

D'Arsonval galvanometer (see Gi, figure 3) used with

the comparator has a free period of 8 sec, an external

damping resistance of 1200 ohms, and a sensitivity

of 5 mm per microvolt at one meter lamp and scale

distance when operated slightly underdamped.

With the Brooks comparator, differences in emf

as large as 2100 /aV may be read with a precision

of 0.1 /iV. The Brooks comparator contains an

opposing circuit for the detection and compensation of thermal emfs in the galvanometer circuit,

contains no sliding parts in the main circuit, and

has a "read-out" device by which the measured

difference in the emf is added algebraically to the

emf of the reference cell thereby giving directly

the entire emf of the unknown cell. A circuit illustrating the principle of the Brooks

comparator is shown diagrammaticaUy in figure 3

and the exterior and interior of the comparator are shown in figures 4 and 5, respectively. The uppermost of the three sections shown in figure 3 relates

to the adjustment of the current in the comparator

to the proper value. A current /i from battery Bi

(No. 6 dry cell) is regulated by the rheostat Ri to a

definite value in the usual manner by reference to an auxiliary standard cell. All parts with exception

of the battery are housed in the comparator. In the middle section, the substitution resistances con- nected to tap points numbered 3 to 16 are selected

by the central dial as needed. Each step is 100 ^tV.

It is necessary to interpolate between successive

steps to complete the process of measuring the emf

of the unknown cell, X (lower section). This is done automatically by the circuits attached to points 17 and 18. B2 and B3 are No. 6 dry cells,

also. The miUiammeter A3 measures the current

I3 when an exact balance is indicated by the galvanometer Gi as a result of varying the resistance R3. The miUiammeter ^3 is calibrated to make its scale

read directly in microvolts. A similar circuit at- tached at 1 and 2 performs a hke function for the

reference cell (lower section) whose emf value

must be set exactly if the readings for the unknown

cell X are to be correctly determined. The lowest

section of figure 3 shows the circuit wherein the un- known, X, and the reference cell, A'^, are connected

in series opposition. This circuit also contains the

galvanometer, Gi, of sensitivity, given above.

Thermal emf compensation is achieved by a simple copper slide wire connected in series with

the main galvanometer, Gi. About 15 (jlA from a No. 6 dry cell (not shown in the figure) enters this slide wire at its central point and leaves by a slider.

If the slider is set at the central point of the copper

wire there is no current in the copper wire and,

therefore, no potential drop. By setting the slider away from the central point, a small adjustable

potential drop may be introduced into the galvaB, R, -^1 ^A/yAAW-|-

COPPER

N STD CELLS X -Hi

Figure 3. Diagrammatic plan of circuits illustrating the principle of the potentiometric part of a Brooks standard-cell comparator.

Figure 4. Photograph of a Brooks comparator (top view).

Figure 5. Photograph of the interior of a Brooks comparator.
nometer circuit to neutralize any parasitic emf in the galvanometer and the wires connecting it to the

comparator. By depressing a "shunt" key any ap- preciable parasitic emf can be detected; if present

it will maintain a corresponding deflection of the

galvanometer and when the "shunt" key is de- pressed the galvanometer coil will assume its opencircuit zero position. To neutralize this undesired

emf the slider is manipulated until no motion of

the galvanometer coil ensues when the "shunt" key

is depressed. For additional details the original

paper by Brooks [46] should be consulted.

Although compensation for thermal effects is provided in the instrument, thermal effects may also

arise at the cell terminals. These are eliminated

by making connections to the cells through mercury

cups immersed two inches below the surface of oil maintained at the same temperature as the cells, as was mentioned above under maintenance (sec- tion 2.3). In those cases (for example, unsaturated

cells or saturated cells in air boxes; see later) where

the terminals of the unknown cell cannot be immersed in oil, thermal effects are kept at a minimum

by using like metal connections in a constant- temperature room. Other types of standard cell comparators have been described by Miller [47], Vincent [48], and by Spinks and Hermach [49]. The comparator (portable potentiometer) of Spinks

and Hermach also includes two saturated and two

unsaturated standard cells in a temperaturecontrolled enclosure.

In the dissemination of the unit of emf, i.e., the

comparison of unknown and reference cells, working

groups of standard cells are used instead of the

National Reference Group of Standard Cells. Of

course, the emfs of the cells within these working

groups are known in terms of the National Reference

Group. These comparisons are made with a precision of 0.6 /iV. One working group is maintained

in the Standard Cell Laboratory in Washington,

D.C., the other at the NBS Boulder Laboratories in Boulder, Colo. [50]. A total of 10 comparisons of

the emf of the unknown (customer) cells with the

working groups is made, one per day, over a period

of ten days, at a specified temperature for saturated

cells and at an ambient temperature of 25 °C (at Washington) and of 23 °C (at Boulder) for unsaturated cells. The best saturated cells are cali- brated with an uncertainty of 0.0001 percent; the

best unsaturated cells with an uncertainty of 0.005

percent. The results are transmitted to customers

in Reports of Calibration.

2.5. International Comparisons

Since 1932 the units of emf of various nations have

been intercompared at specified intervals at the

Bureau International des Poids et Mesures (BIPM),

Sevres, France. At the present time this interval

is 3 years. For many years the comparisons were

rather sporadic, and they were interrupted by the

INTERNATIONAL VOLT

^ France

o Great Britain = Germony

* Japon • Russia

- United States

The National Bureau of Standards provides emf calibrations of unsaturated standard cells only for public utilities and others having operations of such a nature as

lo require calibrations by the National Bureau of Standards.

Figure 6. International comparisons of the unit of electromotive force in international volts prior to 1948. two World Wars (see fig. 6). Prior to 1948 the

intercomparisons were made in "international volts" and since 1948 in "absolute volts" (see figs. 6 and 7). These intercomparisons are effected by standard

cells maintained by the participating countries and

by the International Bureau and are conducted at 20 °C. As a rule each country submits 4 to 10 cells to BIPM for the intercomparisons; at the present

time the cells are carried to BIPM by messenger.

The first comparison involved the measurements

at the National Bureau of Standards in 1910, at which time the units of the participating countries,

France, Germany, Great Britain, and the United

States were identical (see fig. 6). From 1911 to 1932 the major intercomparisons were between the

units as maintained in the United States and in Great Britain. In preparing figure 6 it was assumed

that the mean value for these two nations remained

constant and this mean is made the axis of abscissas.

A few comparisons were also made of the United

States unit with those of France, Germany, Japan,

and Russia and are shown in figure 6 in terms of the mean unit of the United States and Great Britain. In 1931 it was apparent that there was an increasing

discrepancy between the unit as maintained in Germany at the Physikalisch-Technische Reichsanstalt (or PTR) in Charlottenburg and those

maintained in the United States at the National

Bureau of Standards (or NBS) in Washington, D.C.,

and in Great Britain at the National Physical

Laboratory (or NPL) in Teddington. Consequently,

at the invitation of the PTR arrangements were made to have an international committee carry out experiments with the silver coulometer at PTR and

thus reestablish the international volt. Accordingly, P. Vigoureux of NPL and G. W. Vinal of NBS

took silver coulometers to the PTR together with

standard ceDs and standard resistors, the values of

which had been carefully determined at their respective laboratories. At the PTR, these standards were compared with those of the PTR and an

extensive series of experiments with different types

10
of silver coulometers were made. As a result of

these measurements, subsequently published [34,

38, 39], Germany increased its unit by 82 ppm (see

Appendix 4 for United States data).

As a result of extensive work on silver coulometers

by Kolossov [51], Russia changed its unit in 1934 by

87 ppm.

ABSOLUTE VOLT

France

o Great Britain = West Germany

« Eost Germany

* Jopon

• Russia

United States _

e Canada

o Australia

Figure 7. International comparisons of the unit of electromotive force in absolute volts since 1948.

International comparisons at BIPM began in 1932

[52] and after 1934 the data in figure 6 are plotted on the assumption that the "Mean International

Volt" as defined in 1935 by the Consultative Committee on Electricity [53] has remained constant.

In December 1934 the six countries listed in figure

6 submitted cells to BIPM. France, on finding its unit out of line from the others, increased its unit by 76 ppm in 1935. The Consultative Committee

on Electricity suggested that the mean unit of the

five remaining countries be adopted by all thus as- suring immediate unification. However, only

Russia accepted this suggestion and reduced its unit by 13 ppm. In 1937, 1939, and 1945-48, BIPM

carried forward its mean value obtained in 1935 for the six countries after correcting for the Russian

adjustment; this adjustment made the BIPM unit 2.2 ppm lower than its 1935 unit; the data in figure 6

are so reported. For the last comparison in inter- national volts the National Physical Laboratory submitted cells in 1945 and the comparison with BIPM

was completed in 1946; the other countries submitted cells in 1948 and the comparisons with

BIPM were done the same year. Russia reported

its values in absolute units using the international

relation: 1 mean international volt = 1.00034 ab- solute volts. In 1939 the Russian unit had been

23.1 /aV below the mean international unit. For

comparisons with other countries and assuming

that this difference still applied, BIPM converted

the Russian absolute values to international values

using a conversion factor of 1.000317.

On January 1, 1948, "absolute" units were

adopted but international comparisons on this basis were not made until 1950. Intercomparisons in the

"absolute" units, obtained since 1950, are given in figure 7 (note that the scale in figure 7 is much larger

than in figure 6); Australia and Canada were new

additions. In 1950 the German value was from East Germany while in 1953, 1955, and 1957 both East and West Germany took part in the international

comparisons; since 1957 the German value is for West Germany. The data shown in figure 7 are the

deviations from the BIPM mean value. In 1950

Russia had apparently corrected their "absolute"

values of 1948 by the average deviations of the

various countries, Russia excepted, from the BIPM

unit, thus, ignoring the fact that their unit had been

23.1 /LtV below the mean international unit in 1939;

this procedure, then placed their unit in 1950 above

the BIPM unit by approximately this magnitude.

In 1955 Russia made an adjustment in its unit of about 13 ^tV. In 1960 the spread between the 8

countries was 10.2 /xV with the units of Austraha,

Great Brhain, and Russia being high. The spread

between the other 5 countries was 3.3 /jlV. In

1960, values for Italian cells were included with the

values hsted with the French cells; France had first made a direct comparison of the Italian cells with

their cells at the Laboratoire Central d'Electricite,

Fontenay-aux-Roses, France.

3. Early St

Many types of galvanic cells have been proposed

as standards of emf and some of these have ap- peared in a variety of forms. Although Faraday

and others of his time used various galvanic cells, such as Grove and Bunsen cells, in their investigations, the Daniell cell [54] represented by

(-) Zn(s) I ZnS04, H2S04(aq) || CuS04(aq) | Cu{s) (+) was the first cell seriously used as a standard of

emf, as it exhibited less gassing than its predecessors. Here a single vertical fine is used to indicate

the interface of two distinct phases, a double

vertical hne to indicate a liquid-liquid junction,

s = sohd, and aq = aqueous solution. However,

dard Cells

this type of cell did not exhibit a long-term stability in emf. Being a two-fluid cell, the solutions dif- fused into each other causing local action at the

electrodes and a steady decrease in emf. Somewhat better results were obtained when saturated

solutions, no acid, and amalgamated zinc were used,

but, even so, the cells did not exhibit the per- manency required in a standard. Nevertheless,

for over 35 years, Daniell cells were used as a standard of emf. Absolute electrical measure- ments gave 1.07 to 1.14 V for the emf of freshly

prepared cells; the actual value depended on the

concentration and acidity of the solutions used.

Absolute measurements were not needed to show

the lack of constancy in the emf of the cells; this

11
behavior was evident from comparing new cells with old ones. The cell reaction is

Zn(s) + CuSOj (aq) ^ ZnS04(aq) + Cu(s). (4) The Daniell cell has an emf-temperature coefficient

of about +34 /iV/deg C at room temperature (18 to 30 °C).

Daniell cells were made in various forms. The

original form consisted of a glass jar containing a porous pot of unglazed earthenware in which a

zinc plate or rod was placed. Outside and around

the pot a cyhndrical sheet of copper was placed.

The outer jar was fiUed with a concentrated solu- tion of copper sulfate and the porous pot with dilute sulfuric acid, or zinc sulfate, or zinc sulfate acidified

with sulfuric acid [55]. Important modifications were the gravity [56] and Fleming-Thomson cells

[57]. In the gravity cell the less dense solution

of zinc sulfate was placed over the more dense so- lution of copper sulfate so that the porous cup was

eliminated. Zinc in circular or crow-foot form and

copper in leaf form were used as electrodes in their

respective salt solutions. In practice the cell was

kept on closed circuit to curtail the mixing of the

two solutions. In the Fleming-Thomson cell a U-tube was used containing a solution of copper sul- fate in one arm and a solution of zinc sulfate of

the same density in the other arm. Rods of copper

and zinc were supported in their respective salt solutions. In 1872 Latimer Clark [23] proposed a cell which

had a profound effect on work pertaining to the

electrical units. His cell, represented by

(-)Zn(s) I ZnS04 • 7H20(c) | ZnS04(sat aq) | ZnS04

• 7H20(c) I Hg2S04(s) I Hg(/)(+)

where c = crystals, s== solid, / = Uquid, and sat aq = saturated aqueous solution, was a one-fluid

cell free of a liquid-liquid junction and exhibited

relatively good stabihty in emf. Because of the

historical importance of the Clark cell (see sec. 2)

it is discussed in more detail in section 4. Owing

to the success that was obtained with the Clark cell several other one-fluid cells were proposed as standards, the more important being those sug- gested by De la Rue [58], Helmholtz [59], Gouy [60], and Weston [30]. Of these four the first three ex- hibited larger drifts in emf and had higher internal

resistances than Clark cells, while the last one had

many advantages over the Clark cell. As a result,

Weston cells are now used almost exclusively as standards of emf. Weston cells had an emftemperature coefficient about l/30th of that of the

Clark cell, better emf stabihty, and an emf closer

to unity than the Clark cell. Early types of standard cells are listed in abbreviated form in table 2 together with their nominal

emf and emf-temperature coefficient. As is evident

from the table the Weston cell is similar to the

Clark cell except that Cd replaces Zn. In all of

these cells, the original Clark cell excepted, amalgamated anodes were generally recommended and

H-shaped containers (see below) were frequently

used. In general, saturated solutions were used

and excess crystals of the stable salt added to the

cells at the electrode surfaces. Weston [61] and

Carhart [62] proposed the use of an unsaturated

solution of zinc sulfate in the Clark cell in order to decrease its emf-temperature coefficient. The

original Weston patents also covered the unsaturated form of Weston cell. Table 2. Early types oj standard cells

(represented in ubbreviuled fnrm)

Name Date System Approximate

emf, volts Approximate dEjdt,volt per deg C

Daniell 1836 Zn|ZnS04||CuS0j|Cu 1.04-1.14 (nl

-1-0.000034 (n)

Clark 1872 ZnlZnSO^lHgiSOJHg 1.433 (l.S °C) -0.0012 (1.5 °C)

De la Rue 1878 ZnlZnClalAgCllAg 1.03 («) -0.001 (n)

Helmh(jllz 1882 Zn|ZnCU|HgCl|H(! 1.000(15 °C1 + 0.000087 (n)

(iuuy 1888 Zn|ZnSO,|HgO|Hg 1.390 (12 °C) -0.0014 (nl

Weston 1892 CdlCdSO^IHgjSOJHg 1.01864 (20 °C) -0.000041 (20 °C)

'

n represents nominal value at room temperature (about 18 to 30 'O. Somewhat later Bronsted [63] proposed the cell: (—)Pb, Hg(/)|K2S04-PbS04(c)|K2S04(sat aq)|Hg2S04(s)|Hg(Z)(+)

as a standard, stating that at 22 °C it had an average

emf of 1.0481 V and an emf-temperature coefficient

of —0.0001 V/deg C. Henderson and Stegeman

[64], however, stated that the Bronsted ceU did not

exhibit steady emfs with time and that better re- sults were obtained when Na2S04 was used as the

electrolyte. However, their cell at 25 °C had a lower emf of 0.96463 V and a higher emf-temperature coefficient of + 0.000174 V/deg C. These cells have not been used probably because of their high

emf-temperature coefficients. Vosburgh, Guagenty,

and Clayton [65] proposed the cell:

(-)Cd, Bl, Hg(3p)

CdS04-8/3 H20(c). CdSO,Na.S04-2H20(c)sat aq sol of the two salts in 0.1 M

acetic acid or in 0.01 M H-;S0<

HgjS0.(5) Hg(/)( + )

which is a modified Weston cell in which a 3-phase

(3p) amalgam (8.9 percent Cd, 11.1 percent Bi, 80

percent Hg) and a 0.1 M acetic acid or 0.01 M H2SO4

solution saturated with the two salts, as indicated, are used. The cells were sealed with a nitrogen

atmosphere inside. At 25 °C they reported an

average emf of 1.0184 V and an emf-temperature

coefficient of +0.000013 V/deg C which is about 2

to 10 times that of unsaturated Weston cells but

less than one-third of that of the saturated Weston

cell at 20 to 30 °C but of opposite sign. Data avail- able on this cell indicate that it does not have the

long-term stability exhibited by saturated Weston

ceUs, but compares most favorably with the un- saturated type [66].

12
4. The Clark Cell

As stated above, the original Clark cells consisted of a zinc anode and a mercury-mercurous

sulfate cathode in a saturated solution of zinc sul- fate containing crystals of ZnS04 • 7H2O. Clark

used a very simple construction as shown by (a) in figure 8. Various modifications of this general construction were used in commercial cells; one such

modification being the British Board of Trade cell illustrated by (b) in figure 8. In all of these the

zinc anode was in contact with the paste consisting

of mercurous sulfate, a saturated solution of zinc

sulfate, and crystals of ZnS04 • 7H2O.

Clark determined the emf of his cell in absolute

units with (a) a sine galvanometer and (b) an elec- trodynamometer and a B. A. resistor (see footnote 7, p. 4). With these he obtained 1.45735 V and

1.45761 V, respectively, at 15.5 °C, the mean of

which he rounded to 1.457 V as the absolute emf

of his cell. Clark's cell, although much better as a standard than the two-fluid cells, did not show the

permanency in emf hoped for. The cell tended to gas at the zinc anode and the emf showed large

variations owing mainly to concentration gradients

that developed, during slight changes in ambient

temperatures, within the compact paste.

In 1884 Rayleigh and Sidgwick [24] introduced

two modifications in the Clark cell that resulted in substantial improvements. They substituted zinc

amalgam (percentage not stated) for zinc and used

an H-shaped container in which the zinc anode could

be kept entirely under solution and out of contact with the mercurous sulfate paste. Their design is illustrated by (c) in figure 8. The zinc amalgam

reduced the rate of gassing at the anode and by

being under solution and out of contact with the mercurous sulfate paste the zinc anode exhibited more nearly constant potentials. Rayleigh and

Sidgwick obtained 1.453 V for the absolute emf of

their cells at 15 °C using a current balance (now

known as a Rayleigh balance) and a B.A. resistor.

" Rayleigh and Sidgwick presented iheir paper in June 1884; in the paper published

in Dec. 1884 they added the note "Some H-cells have been set up Mr. Threlfall, with amalgams of known composition, varying from 1/32 zinc to 1/5 zinc by weight. The

duration of the test has as yet been scarcely adequate, but it appears that the smaller

quantity of zinc is sufficient." In general, Clark cells were made, at a later date, with 7 to 10 percent amalgams.

sol

(a) (b)

^Hg A-^ ^-Hg A-J^

(c)

sol

^-Hg

(d)

Figure 8. Various forms of standard cell containers.

(a) Original Clark cell Zn — zinc

(b) Board of Trade (British) Clark cell Hg — mercury

(c) Rayleigh Sidgwick H-shaped Weston cell P— mercurous sulfate paste

(d) Sealed H-shaped cell C — cork

(e) Hulett H-shaped cell G — glue

(f) Modified Hulett container sol -solution

(g) Wright-Thompson container A — amalgam

(h) Cooper container S — salt crystals

13

744-338 0-65—

3
These cells tended to leak in time, and Callendar

and Barnes [67] recommended their hermetic seal- ing, (d) in figure 8. Other types of cell containers,

also used for Weston cells, have been proposed and

are iUustrated in figure 8. Hulett [68] proposed a shorter crossarm (e) and today some cells are made

with no crossarm but with a partition at the base of

a single tube (f). Wright and Thompson [69] pro- posed the inverted-Y form, hermetically sealed, and

Kahle [70] the same form but with a cork or groundglass stopper (g); these are more difficult to fiU. Cooper [71] proposed a modified Kahle type (h) which required no support in a thermostaticallycontrolled bath; his form could rest on a flat surface

without support. The Cooper cell could also be

used in water as well as in non-conducting oil because the cell terminals were not exposed but

protruded above the bath fluid. Today, the H-shaped

container, hermetically sealed, is used most widely; some single-tube types, either like (f) or in a modified

form of the original design (a) are also made.

The reaction in the saturated Clark ceU made

with amalgamated anodes is:

j:Zn,i(yHg)(2p) +Hg2S04(s)+—^- (ZnS04-mH20)(sat aq)

= (ZnS04-7H20){c) + 2Hg(Z) + (x- l)Zn, yHg(2p) (5)

where jc = moles of Zn associated with y moles of

Hg in the amalgam, 2p = 2-phase, and m is the number of moles of water associated with 1 mole of

ZnS04 in the saturated solution. Ten-percent

amalgams are most commonly used; they are of two

phases, solid and liquid, with the solid phase being

pure zinc (see fig. 9) [72]. The composition of the

Uquid phase of a 10 percent zinc amalgam at various

temperatures is given in column 2 of table 3. The solubility of zinc sulfate in water changes

considerably with temperature. Above 39 °C the

stable sulfate is ZnS04-6H20; below 39 °C it is ZnS04-7H20. The solubihty of ZnS04-7H20 in water from 0 to 39 °C is given in table 4. In satu- rated solution zinc sulfate hydrolyzes to give a

solution containing 0.004 A'^ sulfuric acid and having

a pH of 3.35 at 25 °C [73]. This concentration of

acid is sufficient to prevent the hydrolysis of the mercurous sulfate used in the positive electrode (see

later). The emf of the Clark cell decreases as the

temperature is increased; the decrease is about 0.1 percent per degree C. For the range 0 to 28 °C

Callendar and Barnes [67] gave, in volts:

E, = Eiy- 0.0012(t - 15 °) - 0.0000062(t (6)

for the dependence of the emf on temperature. At

15 °C and 25 °C this relation gives - 0.00120 V/deg C

and —0.001076 V/deg C, respectively, for dE/dt.

Kahle [74] and Jaeger and Kahle [75] who also studied the temperature dependence of Clark cells obtained results in substantial agreement with

eq (6).

1 1 1 1 1 1 1 1

Liq

Zn + Liq

-

Zn -1- Hg

1 1 1 1 1 1 1 1 "H) 10 20 30 40 50 60 70 80 90 100 Percent Zinc Figure 9. Phase diagram for the zinc-mercury system.

Table 3. Composition of the liquid phases of 10 percent zinc and

cadmium amalgams

Temperature,

°C

Percentage

of zinc Percentageof cadmium

0 1.35 2.50

10 1.60 3.70 20 1.99 5.00 30 2.40 6.40 40 2.90 7.80

Table 4. Solubility o/ZnS04"7H20 in water

Temperature

°C ZnS04 in lOOOg HiO.

grams

0 419

5 446

10 477

15 509

20 542

25 579

30 620

35 663 39° 701

"—transition temperature, 2nS04-7H20 = ZnSO<*6HjO. The Clark cell has three advantages over the

Weston cell, namely, (1) zinc sulfate hydrolyzes to produce sufficient sulfuric acid to prevent the hydrolysis of the mercurous sulfate paste; it, thereby

supplies its own buffering action, (2) the solid phase

of the amalgam is a single component (zinc) whereas

in the Weston cell the sohd phase is a solid solution

of cadmium and mercury; the emf of the Clark cell

is less dependent, therefore, on the composition

of the amalgam than is the Weston cell, and (3) the

Clark cell shows less emf-temperature hysteresis

than the Weston cell. However, the disadvantages

of the Clark cell over the Weston cell well outweigh

its advantages. These are: (1) The Clark cell tends to gas at the amalgam electrode. This gas, as it forms slowly over the surface of the amalgam,

may dislodge crystals of ZnS04 • 7H2O with adhering

solution from over the amalgam, and may eventually cause an open-circuit in the electric circuit unless a

14
constriction, as discussed later, is used in the container at the level of the crystals. (2) Zinc amalgam

slowly alloys with the platinum lead at the base of the amalgam electrode. Since this alloy occupies more space than the platinum, strains are produced,

and breakage of the glass container follows. (3) The Clark cell has an emf-temperature coefficient

which is about 30 times that of the Weston cell.

(4) Owing to the relatively large change in the solubility of zinc sulfate with changes in temperature

crystals of ZnS04 7H2O tend to pass from the

positive to the negative Umb of the ceU. During a temperature rise the volume of electrolyte over the

positive electrode becomes saturated more quickly

because it is usually a smaller volume than that over

the negative electrode. As a result a concentration

difference between the limbs results and diffusion

of electrolyte to the negative hmb takes place until the concentration difference is dissipated. This

phenomenon can be prevented by making the vol- ume above the negative and positive electrodes

equal. A similar phenomenon does not occur in 5. The Weston (or C

5.1. General

The Weston (or cadmium sulfate) cell is, for all intents and purposes, the only electrochemical

system used today as a standard ceU. Accordingly,

the words "standard cell" when used today in- variably mean cadmium sulfate cells, and the re- mainder of this Monograph, appendices excepted,

relates to this type of cell. The cell is made in two general types, saturated and unsaturated where

these terms refer to the state of the electrolyte.

The saturated type is the precision cell used in the

maintenance of the unit of emf. It may be made

in a highly reproducible form and exhibits a constant emf for long periods of time. However, for high precision it must be maintained at a constant temperature owing to its relatively high emf-temperature coefficient. Most saturated cells must

be hand carried, although some recent types have

been made shippable by locking the electrodes

in place with inert and porous septa. The un- saturated cell is less stable than the saturated type

in that its emf decreases slowly with time and is regarded as a reference of d-c voltage known within

±0.005 percent. It is usually made in a shippable

form with a septum over each electrode. It has

a low emf-temperature coefficient and accordingly

is used widely in ambient temperatures as an emf

reference where 0.005 percent (0.05 mV) accuracy

suffices. The saturated cell is also known as the Weston

Normal CeU (or Element). It consists of a cadmium

amalgam anode and a mercury-mercurous sulfate cathode in a saturated solution of cadmium sulfate with crystals of CdS04 • 8/3H2O over the surface

of both electrodes. This cell may be represented

by:

(-)Cd, Hg(2p)|CdS04-8/3iH20(c)|CdS04(sat aq)| _ CdS04-8/3 H20(c)|Hg2S04(s)|Hg(;)( + )

Weston cells because the change in solubihty of

cadmium sulfate with temperature is much less than that of zinc sulfate. (5) The Clark cell has an emf which is considerably higher than unity

which, although not necessarily a disadvantage,

makes its use as a standard less convenient than a Weston cell. The emfs of saturated Clark cells are decreased

by addition of sulfuric acid to the electrolyte.

Some data based on measurements of Hulett [76] are given in table 5. Table 5. Effect of sulfuric acid on the emf of Clark cells at 25 °C

Normality of sulfuric acid °

Change in emf. Before saturation After saturation with ZnSO, • 7H2O with ZnSO, THiO

loll 0.2024 0.1074 -0.00028

1.012 0.5400 -0.00233

2.024 1.1488 -0.0O448

" — normality of acid in a saturated solution of pure zinc chloride at 25 °C is 0.0O4

.V [73]. dmium Sulfate) Cell

where the symbols have the same significance as given above. The cell reaction is:

xCd. rHg(2p) + Hg2S04(5)+ —^(CdS04 TTiHsOXsat aq) m — 0/0 = —^

(CdS04 • 8/3H20(c) +2Hg(^) -^(x- DCd, yHg(2p). (7) m — 8/3 where x moles of Cd are associated with y moles

of Hg in the amalgam and m is the number of moles

of water associated with 1 mole of CdS04 in the

saturated solution. At the end of the reaction the

amalgam may be two phases (liquid and solid) or may be a hquid phase only, depending on the ex- tent of the reaction and the relative amounts of

amalgam and mercurous sulfate used in preparing

the cell. When not discharged, as is normal when

the cell is used as an emf standard, the amalgam

remains in two phases. Cells are usually made

with 10 or I2V2 percent amalgams (see later). The unsaturated cell differs from the saturated

type only in that an unsaturated solution of cadmium sulfate and no crystals of CdS04 • 8/3H2O

are used. It is customary to use a solution that is saturated at 3 or 4 °C, the temperature range at which the salt exhibits a minimum solubihty; the

solution is then unsaturated at higher temperatures.

The emf of the unsaturated cell at ambient room

temperature is about 0.05 percent higher than that of the saturated type. The ceU reaction is simply:

xCd, jHg(2p) + Hg2S04(5) = CdS04(aq)

+ 2Hg(/) + (z-l)Cd,yHg(2p) (8)

unless the reaction is continued untU crystals of CdS04 • 8/3H2O are formed: then the reaction is the same as in the saturated ceU. Again, when

the ceU is not discharged, as is normal when the

ceU is used as an emf standard, the solution remains

unsaturated and the amalgam in two phases.

15
Both saturated and unsaturated standard cells have been made as "neutral" or as "acid" cells; the

saturated "neutral" type is also known as the Weston

Normal Cell. These terms refer to the degree of

acidity of the electrolyte with respect to sulfuric acid

in the cell. If an aqueous solution of pure cadmium

sulfate, to which no sulfuric acid is added, is used

in the preparation of the cell, the cell is called a "neutral" type, even though the pH of a saturated

aqueous solution of cadmium sulfate, owing to hydrolysis, is 4.00 at 25 °C [73]. If sulfuric acid is added in sufficient amount to make the acidity 0.03

to 0.1 A^, the cell is called an "acid" type. The

purpose of adding the acid to the electrolyte is to prevent hydrolysis of the mercurous sulfate used in the cell; more details are given later. Today, most,

if not all, cells are made of the "acid" type.

5.2. Preparation and Properties of Materials

The procedures employed at the National Bureau

of Standards for the preparation and purification

of materials for use in the construction of standard

cells are described in the next few sections. In

addition some properties of the materials, as they

relate to standard cells, are given. Only four materials are required. These are mercury, cadmium,

sulfuric acid, and water, all of which may be purified by distillation. To these four materials may

be added a fifth, CdS04 • 8/3 H2O, to avoid preparing

it from cadmium and sulfuric acid; it may be purified by repeated crystallizations from conductivity

water. Mercury of good grade, after washing in dilute

nitric acid, then in distilled water, is dried and

then distilled in a Hulett [77] still in a stream of

dry air. In this method, the mercury is distilled near 200 °C in a partial vacuum, air being drawn

through the condenser at a rate to maintain the

air pressure at 25 mm, corresponding to an oxygen

partial pressure of 5 mm. Any metallic vapor, except mercury, is oxidized and collects as a scum

on the distillate which is removed by filtering

through a fine pinhole in filter paper. Metals more

noble than mercury remain in the boiler of the still. Finally, the mercury is redistilled in a vacuum

still. Procedures for purification of mercury of

various grades are outlined in reference [78]. Cadmium of electrolytic grade is sublimed under

reduced pressure. The sublimation is done at about 350 to 400 °C in an evacuated Pyrex glass

tube with an external electrical heating jacket.

The distilled cadmium crystallizes out on the cooler

parts of the tube but frequently adheres tightly

to the glass walls. It is usually necessary to break

the glass walls to remove the distilled cadmium

adhering to the walls. Sulfuric acid of reagent grade is twice distilled

in an all-Pyrex still at a temperature of 270 to 290 °C

with the middle fraction being retained. Glass

beads are placed in the boiler to reduce bumping

and the distillation is carried out in a hood.

Water is repeatedly distilled in a Barnstead or comparable still until the conductivity of the water

becomes as low or lower than 1 X 10~^ ohm~* cm~^

This water is frequently called "conductivity water." Cadmium sulfate (CdS04 • 8/3 H2O) of high grade

is obtained by several recrystalhzations of the CP.

salt. These recrystalhzations are carried out from

aqueous solutions below 43.6 °C [79] since above

this temperature the stable salt is the monohydrate,

CdS04 • H2O. Cadmium sulfate of high grade may

also be prepared from pure cadmium and redistilled sulfuric acid solutions or from pure cadmium

nitrate and redistilled sulfuric acid solutions,

followed by recrystalhzations of the salt from

aqueous solutions in each case. The latter method

is preferred rather than the use of very impure

cadmium sulfate as starting material.

Both hydrates, CdS04 • 8/3 H2O and CdS04 • H2O,

are highly soluble in water; the solubihty of the for- mer increases with temperature while that of the

latter decreases. The solubihties of each as a function of temperature are given in table 6 [79, 80,

81]. The solubilities are expressed as the number

of grams of the anhydrous salt in 1000 g of water. Saturated solutions of CdS04 • 8/3 H2O begin to freeze at — 17 °C and become completely frozen at — 24°C; these temperatures, then, represent the

lower hmit for the use of cadmium sulfate cells (see

later). It has been reported [82] that CdS04 • 8/3

H2O shows a transition to CdS04 • 7H2O at 4 °C. However, cells tested in this temperature range do

not exhibit an abrupt inflection in emf at 4 °C indicative of a phase change but a smooth maximum in the range of 3 to 4 °C indicative of a minimum sol- ubihty in cadmium sulfate (see table 6). The amalgam may be prepared either electrolytically by depositing cadmium in mercury or by

heating the two metals together. In the former,

crystals of purified CdS04 • 8/3H2O are placed on

the surface of mercury (contained on the bottom of

a beaker) and covered with distiUed water acidified

with a few drops of sulfuric acid. Mercury is made

Table 6. Solubility of CdS04 • 8/3H2O and CdS04 • H2O in aqueous solution

(given as grams of CdSOj in 1000 grams of water)

Temperature,

°C CdS04 • 8/3 HaO (as CdSO,) CdSO, • HjO(as CdSOJ

-18 . 767.6 -15 765.3 -10 762.1 - 5 759.2 0 757.5

3 757.2 5 757.5

10 759.4

15 761.4 20 763.7 25 766.9 30 771.0 35 776.5 40 783.8 42 787.2

43.6 ' 790.5 45 793.5 786.5 50 806.1 770.6 55 754.7 60 738.9 65 722.0 70 707.0

72 700.0

" — transition temperature.

16
the cathode and a platinum foil suspended in the

acidified solution at the top serves as anode. Usually only the amount of CdS04 • 8/3 H2O needed to prepare an amalgam with the desired percentage

of cadmium is used. Since the crystals dissolve

slowly, a current of low magnitude is used initially until nearly all the cadmium is deposited. The

current is then increased to about 5 times the initial value for 30 minutes, and then, with the current

still flowing, washing with distilled water is started. Water is successively added and decanted until the current decreases to less than 0.01 A for an impressed voltage of 110 V. The remaining water is then siphoned off and the last drops of water re- moved from the surface of the amalgam by filter paper. A cadmium rod may be used as anode in an alternative procedure; the amount of cadmium

deposited in the mercury is determined from the current and time by Faraday's law.

The method in which the two metals are heated

together is more convenient. The proper weights

of the two metals are heated together in a covered

porcelain casserole, in a hood, until completely

melted and homogeneous. A shght scum may

form on the surface. This is brushed aside and

the hquid amalgam immediately transferred to an

electrically heated covered buret and introduced

directly to the cell through an electrically heated

dehvery tube. Any small amount of scum on the

surface of the amalgam will disappear as soon as

acidified cadmium sulfate is added to the cell. This

is the method now used at the National Bureau of

Standards.

No difference in the properties of standard cells containing amalgams prepared by these methods

has been observed.

Amalgams exhibit less erratic behavior and cor- rode less rapidly, if at all, than pure unamalgamated

metals when in contact with aqueous solutions of

their salts. Amalgamation raises the hydrogen

overvoltage of metals and reduces stresses within

the metal. It is for these reasons that amalgams

make better anodes for standard cells than the

pure metals. A 10 or I2V2 percent amalgam is generally used commercially in constructing cadmium sulfate cells. A 10 percent amalgam is now

used by the National Bureau of Standards. These

are weight percentages. At normal temperatures

amalgams of these precentages consist of two

phases, a hquid and a solid phase. The solid phase

is a sohd solution of cadmium and mercury. The

phase diagram [83] for the cadmium-mercury sys- tem is given in figure 10. The composition of the

hquid phase of a 10 percent amalgam is hsted for various temperatures in column 3 of table 3. Mercurous sulfate may be prepared in a number

of ways. Shortly after Weston patented his cell much research was devoted to methods of preparing

and storing mercurous sulfate and cells made with

various preparations were extensively studied.

Some samples were white, i.e., devoid of free mercury while others were gray in appearance owing

to the presence of finely dispersed mercury. Some

samples were thoroughly washed with absolute

alcohol and anhydrous ether and dried in vacuo

while other samples were kept moist with either

dilute sulfuric acid or saturated solutions of cadmium sulfate acidified slightly with sulfuric acid. Mercurous sulfate may be prepared in a number

of ways as follows:

(1) By the reaction between sulfuric acid and

mercurous nitrate [84].

(2) By the action of fuming sulfuric acid on mer- cury [84].

(3) By the action of dilute nitric acid in sulfuric acid on mercury [44, 84].

(4) By the reduction of mercuric sulfate by mercury [44].

(5) By the reduction of mercuric sulfate by sul- furous acid [44].

(6) By the reduction of mercuric sulfate by formaldehyde [85].

(7) By the recrystaUization of commerical mer- curous sulfate from sulfuric acid [84].

(8) By a-c electrolysis [86].

(9) By d-c electrolysis [44, 68]. Of these methods standardization has been on

the last one which is now used by the National

Bureau of Standards. By d-c electrolysis uniform

samples of mercurous sulfate of high purity and

reproducible grain size are obtained. Such samples are free of all foreign materials except sulfuric acid, water, and mercury, all of which are used in standard cells, and especially in the mercurous sul- fate paste. In this method [44, 68] mercury anodes,

platinum cathodes, and a 1:6 sulfuric acid-water so- lution are used and electrolysis is carried out in a darkened room (see sec. 7.7). Mercury is placed on the bottom of one or two shallow glass vessels. These vessels are then placed on a glass stand one above the other, as shown in figure 11, in a deeper

and larger dish ^/s filled with the sulfuric acid so- lution; the acid solution extends over the shallow

vessels. The upper dish contains a central glass

tube through which passes the shaft for the stirrer for the lower dish; obviously this is unnecessary if only one shallow dish were used. A platinum-foil

cathode is placed near the top of the solution. The

solution is vigorously stirred at 70 to 200 rpm and

<u 200

Q. 100 E

1 1 1 1 1 1

Liq

\ (3(Hg,Cd)

oc(Cd,Hg)

1 1 1 1 /

/ »c+ P

1 1

1 l\l

t) 10 20 30 40 50 60 70 80 90 100 Percent Cadmium

Figure 10. Phase diagram for the cadmium-mercury system.

17
Figure 11. Views of the electrolytic preparation of mercurous sulfate.

Left vessel— stirrer in operation, cloudiness due to mercurous sulfate suspended in solution. Center and right vessels — stirring stopped, mercurous sulfate at bottom of vessels and on

surface of mercury in dishes.

the current density at the mercury surface is maintained at 1 to 2 A per 100 cm 2. When the electrolyte becomes saturated with mercurous sulfate the

soHd salt appears on the surface of the mercury

and must be swept off into the outer dish by stir- ring in order to keep the surface of the mercury

anode clean. In this stirring finely divided mercury

becomes mixed with the mercurous sulfate to give

a gray product. Electrolysis is continued until the

desired amount of gray mercurous sulfate has been

prepared; the current is then cut off but stirring is continued for several hours. Photographs of the

electrolytic production of mercurous sulfate are shown in figure 11. For "neutral" cells the mercurous sulfate is then

washed repeatedly with a saturated solution of

cadmium sulfate and then stored under such a

solution until used. For "acid" cells the mercurous

sulfate is stored under the electrolysis solution

until needed at which time it is washed thoroughly

with the solution of the type to be used in the cells. Mercurous sulfate is the oxidizing agent or the

"depolarizer" used in Weston cells. It is highly

insoluble in water and in dilute solutions of sul- furic acid. The solubility of mercurous sulfate,

expressed in terms of mercury, in various concentrations of sulfuric acid is listed in table 7 at 0 and

28 °C [87]. In aqueous solutions mercurous sulfate hydroiyzes according to the reaction:

2Hg2S04(s) +H20(/) = Hg20 • Hg2804(5)

+ H2S04(aq) (9)

to form a basic salt, Hg20 • Hg2S04, and an equilibrium amount of sulfuric acid. Gouy [88] and

Hager and Hulett [89] found this concentration

to be 0.002 while Craig, Vinal, and Vinal [87]

showed that it did not change appreciably with

temperature, being 0.00198 at 0 °C and 0.00216 N

at 28 °C. Although, theoretically, a concentration

shghtly exceeding the equilibrium concentration

could be used to prevent the hydrolysis of mer- curous sulfate, a higher concentration is recom- mended. In initial studies [90], solutions 0.03

to 0.06 with respect to sulfuric acid were used,

in part because mercurous sulfate exhibits a minimum solubility in the range from 0.04 to 0.08 A^. (Hulett [91] and Sir Frank Smith [92] found the

minimum solubility of mercurous sulfate to be at approximately 0.04 A^, while Craig, Vinal, and Vinal

found it to be at 0.06 A^ at 28 °C and 0.08 A^ at 0 °C.)

Now, the lower acidity value is preferred since cells with the lower acidity have shown the greater

stabiUty in emf. Cells with 0. 10 A'^ acid, for example,

tend to gas as a result of the action of the acid with

the cadmium amalgam.

Concentrations of sulfuric acid somewhat higher

than the equihbrium value are chosen, not only

because of the decrease in the solubihty of merTable 7. Solubility of mercurous sulfate in aqueous solutions of sulfuric acid

Molarity of HiSO,

Grams of Hg per 100 ml

Molarity of HjSO,

Grams of Hg per 100 ml

( = 0°C t = 2S°C f = 0°C 1 = 28 "C

0.002 0.0290 0.0463 0.100 0.0183 0.0344

.004 .0239 .0395 .200 .0198 .0379

.006 .0215 .0360 .300 .0212 .0403

.008 .0203 .0346 .400 .0224 .0423

.010 .0197 .0338 .500 .0233 .0438

.020 .0182 .0318 .600 .0239 .0451

.030 .0179 .0313 .700 .0244 .0461

.040 .0178 .0317 .800 .0247 .0467

.050 .0178 .0322 .900 .0248 .0470

.060 .0178 .0327 1.000 .0249 .0470

.070 .0179 .0332 2.000 .0240 .0409

.080 .0180 .0337 3.000 .0139 .0294

.090 .0181 .0341 4.000 .0078

18
curous sulfate but because the acid decreases the

rate of solubility of the glass container by the electrolyte. It is well known that glasses are more

soluble in neutral salt or alkaline solutions than in acidic ones [93, 94]. The final solution is titrated against sodium hydroxide using methyl purple as an indicator, which has a pH transformation interval

of 4.8 to 5.4 [95]. 5.3. Containers for Standard Cells

Saturated standard cells as made at the National

Bureau of Standards are of the H-form shown in figure 12. Photographs of a cell container and a completed cell constitute figure 13. The container

is made of Kimble Standard Fhnt glass, a chemically resistant soda-Hme glass, having an average

linear thermal expansion coefficient of 92x10"^

per degree C. (The coefficient of linear thermal

expansion is the increase in length per unit length,

measured at 0 °C, per degree Celsius.) Since this linear thermal expansion coefficient approximates

that of platinum, 89X10"^ per degree C, vacuumtight seals are obtained at the platinum leads. No lead-containing seahng-in glass is used in sealing in the platinum leads at the bottom of each

limb as the solution may extract the lead leading

to cell deterioration. On the average the height

of the cell is about 92 mm, the diameter of the vertical hmbs about 16 mm, the diameter of the cross- arm about 11 mm, and the distance between limbs

about 22 mm. A constriction is made near the

base of both limbs, as shown, to lock in part of the

crystals of CdS04 • 8/3H2O. The constriction

may be a complete circumferential indentation or may consist of several knobs directed inward.

The constrictions are so placed that crystals of

CdS04 • 8/3H2O are both below and above them.

These locked crystals prevent the displacement

of the materials in the cell hmbs, and those at the

anode help prevent an opening of the circuit within

the cell by any gas that might form on the amalgam.

The platinum leads are secured by cotton thread

to the side of each hmb at the constrictions and

cemented in place by collodion; platinum leads

have a tendency to break mechanically at the glass

seal if not securely held in place. For permanent

records a number is etched on each cell on the

outside wall of the container with hydrofluoric acid. Prior to fiUing with cell constituents the container

is thoroughly cleaned with nitric acid, rinsed with

distilled water, steamed, and then dried at 110 °C. At various times attempts have been made to use Pyrex for cell containers. However, since

Pyrex has an average hnear thermal expansion

coefficient of 32 X 10~^ per degree C tungsten which

has a linear thermal expansion coefficient of 43x IQ-^

per degree C must be used for external electrical connections to assure vacuum-tight seals. Tungsten, however, is more brittle than platinum and a number of cells have had to be discarded because

of breaks at the tungsten-Pyrex seal. Fused sihca, owing to its extreme inertness, has

frequently been suggested for standard-cell conCd SO^-S/JHjO'

10% Cd AMALGAM

SATURATED SOLUTION

CdS04-8/3H20

HggSO^

Figure 12. Sketch of a saturated standard cell of the cadmium

sulfate type (National Bureau of Standards type).

Figure 13. Photograph of container and completed saturated standard cell of cadmium sulfate type (National Bureau of Standards types). tainers. Fused sihca, however, has two major

drawbacks. It has such a low linear thermal ex- pansion coefficient (5 X 10"^ per degree C) that the

electrical leads have to be brought into the cell in a special way. Also, a very high temperature must

be employed to seal the cell; the chemicals within

the cell may thereby be affected. These two drawbacks have been solved by the cell design [96] shown

in figure 14. The main features of this design are

(1) the use of two seals at each arm and (2) a graded

seal between the fused sihca and Pyrex at the top

of each hmb of the container. The use of the

graded seals makes possible the seahng of the cell

at a low temperature after filling. Transparent

sihca is used to facihtate the filling of the cells. The two seals at each arm are spaced about 7 cm

19
-J 1.6 cm i— -4 1.6 cm L

'

I A-PYREX

B - GRADED SEAL

C - TRANSPARENT FUSED SILICA D - SEALED WITH FUSED SILICA A

6 cm SHRUNK AROUND PLAT- INUM WIRE ""28 E- VACUUM TIGHT SEAL

Figure 14. Cross-sectional sketch of standard-cell container made offused silica. apart. The lower seal consists of platinum wire

No. 28 sealed in silica, and although this seal is not vacuum-tight it is sufficiently tight to retain

the cell constituents. The upper vacuum-tight

seal is a Houskeeper [97] type and consists of

molybdenum ribbon sealed in silica (this type of

seal cannot be used as the lower seal since molybdenum is chemically attacked by the cell constituents). The molybdenum ribbon and platinum

wire are welded at a point about 2 cm below the

upper seal. Sihca is shrunk around the weld, the

molybdenum ribbon, and platinum wire. Copper

terminals are used and are silver-soldered to the

molybdenum ribbon. Errors that might arise from

thermoelectric effects are prevented by completely

immersing the cells in oil at a constant temperature. Dimensions of the cell are included with

the figure.

Although it has been generally thought that inter- actions between the glass container and the cell electrolyte cause "aging" or drifts in the emf of

standard cells, available data have not shown sig- nificant differences between cells made in soft glass, Pyrex, or fused sihca on a long-term basis. Plastic containers have also been proposed [98]. These must prevent vapor transport or a drying out

of the cells may occur.

5.4. Assembly and Mounting of

Standard Cells

Two long-stemmed funnels, one sliding through

the other, are employed at the National Bureau of

Standards for introducing materials into the container. The inner funnel carries the material and

the outer one serves to prevent the material from

coming into contact and "mussing up" the walls

of the container. After introducing the material

the stem of the inner funnel is drawn up into the

stem of the outer funnel; both funnels may then be

withdrawn from the ceU without any of the material

touching the container walls. Mercury is first placed in the bottom of one limb

and the amalgam in the bottom of the other hmb,

each to a depth of about 6 mm. The amalgam is added while warm and in a single hquid phase; on

cooling the amalgam becomes two-phased, solid and

hquid. The mercurous sulfate is then placed in a Gooch or similar crucible, washed free of the solu- tion under which it was stored with dilute sul- furic acid and then washed with solution of the

type to be used in the cell, and then while moist

introduced over the mercury to a depth of about 13 mm. The mercurous sulfate should be mixed with

a small amount of mercury (partially done during

the electrolytic preparation of mercurous sulfate)

and finely divided crystals (fineness of granulated

sugar) of CdS04 • 8/3H2O prior to introduction to the cell; this mixing may be done prior to the washing procedure. The mixing and washing of the mercurous sulfate paste hastens the attainment of

chemical equihbrium within the cell after its assembly.

Crystals of CdS04 • 8/3H2O, of a size that will pass through a tube of 4-mm bore, are then added

to both limbs of the ceU to a depth of about 10 mm

at the negative electrode and about 8 mm at the

positive electrode. Finally, a saturated solu- tion of CdS04 • 8/3H2O is added to a level shghtly

above the crossarm, and the cell is then hermetically

sealed. In some cells, especially of larger size, large

crystals (about 10 to 15 mm in diameter) of CdS04 • 8/3H2O are used. Larger crystals have an advantage over smaller crystals in that any gas that may

form at the electrode surface (especially the negative one) will not become entrapped by the crystals

whereby an open circuit might be produced. However, cells with large crystals tend to come to equilibrium, after a temperature change, more

slowly than those made with small crystals.

Unsaturated cells are made similarly except that no crystals of CdS04 • 8/3H2O are used and they

are made portable (shippable) by inserting cork

or plastic rings, covered with hnen, over the elec- trode surfaces. In some cells ceramic discs, either

locked in place or supported by ceramic rod which

protrudes through stoppers in each hmb, are used

[99,100]. The unsaturated cell is the commercial

type used widely in the United States for work

requiring no greater accuracy than ± 0.005 percent;

it is not made at the National Bureau of Standards.

It is used for pyrometer work, in pH meters, re- cording instruments, etc., and is usually housed

in nontransparent copper-shielded cases for general

laboratory work. A copper-shielded case is employed to assure a uniform temperature at both

hmbs of the cell (see later). Saturated cells are

not mounted in cases since they are intended for immersion in temperature-controlled oil or air baths. Commercial saturated cells are usually

mounted in groups of 3, 4, or 6 on special racks

20
for convenience in use. NBS saturated cells are mounted in oil as described above in section 2.3. In practice, saturated standard cells are maintained at a constant temperature in thermostatically controlled oil baths or in portable thermostatically controlled air boxes. The latter are

generally made after a design first proposed by

Mueller and Stimson [101]. The cells are housed

in a thin-walled aluminum box which rests in a larger

thick-walled aluminum box, the temperature of

which is controlled by a mercury-in-glass thermoregulator. The aluminum boxes are thermally in- sulated and are enclosed in a wooden box which

also contains an a-c relay, a transformer, and a pUot light. The box is operated on 110 V 60-cycle

a-c hne. The leads from the individucd cells are brought to binding posts on the outside of the box.

These boxes are designed to operate at temperatures above room temperature; the choice of temperature depends on the location where the boxes

will be used or on the size of the box. As a rule these boxes operate at some temperature between

28 and 37 °C. The temperature of the cells in the air box is measured by a mercury-in-glass

thermometer provided with the box. In some

boxes, a well is added to provide for a platinumresistance thermometer in which case the temperature is so measured. A detailed description of a MueUer-Stimson box is given in Appendix 7.

5.5. Electromotive Forces of Newly

Made Cells

Cells when first made will, except in the most

rare cases, exhibit emfs that wiU differ considerably from the final steady values. Values steady

to 0.01 mV or 10 /xV are usually obtained within a few days but one to three years may be required

before values steady to 0.1 ^tV are attained. In some cases the emfs decline in their approach to equihbrium, in other cases they increase. There

are two main reasons for this "aging" process, namely, (1) equalization of acid throughout the cell and especially within the mercurous sulfate paste,

and (2) attainment of solution saturation. Although

attempts are made to take care of both of these

during cell assembly, inequalities in acid concentration may readily occur during the filling process

owing to evaporation; and since the preparation of

solutions of cadmium sulfate which are truly sat- urated is an extremely slow process unsaturated

solutions may inevitably be used in preparing cells. Cells that "age" with a decrease in emf probably

have been made with unsaturated solutions, even

though crystals of CdS04 • 8/3H2O are present, or have too much acid in the negative limb while those

that increase in emf during "aging" probably have

excess acid in the positive limb. Also, the reaction

between the electrolyte and the glass container

probably contributes to "aging" since as the glass neutraUzes some of the acid in the cell the emf wiU

increase. However, this reaction eventually ceases

as is shown by the emf stability of cells which have

been "aged" (see later). If reproducibility of 0.01 mV in emf is considered adequate, all of these fac- tors are inconsequential.

Cells within any one group, although made at the same time of the same materials, may have emfs

that differ by as much as 0.005 mV (or 5 //.V). These differences may not only be present initially

but may persist for years; i.e., individual cells,

although differing in emf may show high stability in emf. Although diffusion is a slow process, acid or

solution concentration inequalities within any one

cell cannot explain these differences. Instead they

must arise (1) from slight differences in the composition of the amalgam between cells (see fig. 15,

especially the horizontal sections; this figure is discussed in sec. 6.3) even though precautions are taken to keep the amalgam homogeneous during

the filling process, (2) from slight differences in the

acidity of the solutions between cells produced

during the filling process or in interactions with the

glass containers, and/or (3) to slight differences

between cells in the grain size of the mercurous

sulfate used in the positive electrodes.

Percentage of cadmium

Figure 15. Relation of the electromotive force of the cadmium

sulfate standard cell to the percentage of cadmium in the amalgam and the temperature.

21
6. Effect of Variations in Components on the Electromotive Force of Standard

Cells

6.1. Concentration of Solution

The emf of cadmium sulfate cells depends on the

concentration of the cadmium sulfate solution.

Over a limited range of concentration near saturation the emf increases about 0.0017 V for a one

percent decrease in cadmium sulfate content. For more dilute solutions the change is somewhat

greater. In table 8 the emfs at 25 °C corresponding

to various concentrations of cadmium sulfate, near

saturation, are given. These data are based on

results of Vosburgh and Eppley [102] for cells containing 12 V2 percent amalgams and 0.023 A^H2S04;

their data reported in international volts have been

converted to absolute volts here. For neutral solu- tions the emfs listed here would be 18 fxY higher.

Table 8. Electromotive forces of cadmium sulfate cells at 25 °C

as a function of electrolyte concentration

CdSO. Emf CdSO. Emf

percent V percent y

41.84 1.021289 42.98 1.018990

42.39 1.020019 43.06 1.018883 42.63 1.019673 43.12 1.018704 42.77 1.019315 43.22 1.018603 42.90 1.019121 43.402 • 1.018392 42.94 1.019043

' —saturated solution.

6.2. Acidity of Solution

The addition of sulfuric acid slightly decreases

the emf of a cadmium sulfate cell. For the satu- rated cell several equations have been proposed

relating the change in emf to acid concentration.

For acid concentrations up to 4 Sir Frank Smith

[103] gave:

A£(volts) =- (0.00060;c + O.OOOOSjc^) (10)

where AE is the difference in emf of acid cells from

neutral ones and x is the normahty of the sulfuric

acid solution before it is saturated with CdS04-8/3

H2O. For acid concentrations up to 0.4 only, the

National Physical Laboratory [104] gave the linear

relation

A£'(microvolts) =— 615:c. (11)

Obata [105] and Ishibashi and Ishizaki [106] also gave linear relations with the coefficients being,

respectively, —855 and — 833; Vosburgh [107] found

Obata's equation to be vahd to 1.49 A^. In these

last two, X refers to the normality of the acid in a

saturated solution of CdS04-8/3 H2O. AH of these

formulas agree closely if applied properly. For low

acidities, the acidity of a solution of sulfuric acid

after saturation with CdS04-8/3 H2O is 0.767 of that before saturation.

22

Differences between the emf of "neutral" and

"acid" cells according to the Smith formula are Usted in table 9. Sir Frank Smith [103] also investigated the effect on the emf of cadmium

sulfate cells if the acid were confined to one or the

other of the electrode compartments. For these

effects, valid to 4 A'^, he gave:

A^negatlve(volts) = 0.01090x - 0.00125^2 ( j2)

A£positive(volts) = - 0.01 150a: + 0.00120a; ^ (13)

the summation of which gives the Smith equation

above for the effect of acid on the emf of the cell as a whole. These differences for various normalities of acid are given in table 10. These relations

show that more acid at the negative electrode

increases the emf while more acid at the positive

electrode produces a decrease in emf. It is necessary, then, for high reproducibility and stability in emf that the acidity be the same and remain the same at the two electrodes.

Table 9. Differences of emf of "acid" cells from the standard

value of Weston "neutral" cells

Normality of H,S0.

DifTerence of emf

Normality of H,S0,

Difference of emf

0.01 i^y -6 0.30 -IBS

.02 -12 .40 -248

.03 -18 .50 -312

.04 -24 .60 -378

.05 -30 .70 -444

.06 -36 .80 -512

.07 -42 .90 -580

.08 -48 1.00 -650

.09 -54 2.00 -1400

.10 -60 3.00 -2250

.20 -122 4.00 -3200

Table 10. Differences of emf of cells from the standard value of Weston "neutral" cells if the acid is confined to the negative or positive limb of cell

Differences of emf. microvolts Normality of H,SO,

Negative limb Positive limb

0.01 + 109 -115

.02 + 218 -230

.03 + 326 -344

.04 + 434 -458

.05 + 542 -572

.06 + 650 -686

.07 + 757 -799

.08 + 864 -912

.09 + 971 -1025

.10 + 1078 -1138

.20 + 2130 -2252

.30 + 3157 -3342

.40 + 4160 -4408

.50 + 5138 -5450

.60 + 6090 -6468

.70 + 7018 -7462

.80 + 7920 -8432

.90 + 8798 -9378

1.00 + 9650 - 10300

2.00 + 16800 -18200

3.00 + 21450 -23700

4.00 + 23600 -26800
6.3. Composition of the Amalgam

The emf of cadmium sulfate cells depends on the

composition of the amalgam as may be seen by the

data (converted to absolute volts) of Sir Frank Smith

[108] and shown plotted in figure 15. This figure

clearly shows that the useful range of amalgam composition is from about 8 to 14 percent cadmium for normal temperatures. The useful range of amalgam

composition for standard cells is limited to the

horizontal part of the curves; here the emf is very

insensitive to or not critically dependent on the exact percentage of cadmium in the amalgam. In

this range the amalgam consists of two phases,

solid and Uqiiid, with the solid phase being a sohd

solution of mercury and cadmium. For low and

high percentages of cadmium the emf is very sensitive to the exact amount of cadmium in the amalgam; this sensitivity is more marked at lower temperatures. In figure 16 is shown the range over which amalgams of various cadmium content may

be safely used. The range for use for a I2V2 per- cent amalgam is about 12 to 62 °C while for a 10

percent amalgam the range is from about —8 to 51 °C. It is for this reason that a 10 percent

amalgam is used in cells made at the National

Bureau of Standards. The significances of the

lower dotted lines A and B appearing in figure 16 were discussed under the section on cadmium

sulfate; the significance of the upper dotted hne is discussed in the next section.

6.4. Crystal Phases of Cadmium Sulfate

As was stated above, cadmium sulfate over a normal temperature range exists in two different

hydrates, CdS04 • 8/3H2O and CdS04 • H2O with the

transition temperature being 43.6 °C, below which

the former hydrate is the stable form. The emfs

of saturated standard cells made with these two

different hydrates differ except at the transition

point where the emfs are the same. Cells can be

used in the metastable range for short periods of

time. In either case, at a particular temperature,

cells made with the more stable hydrate have the

higher emf. These facts are illustrated by the data

[109] given in table 11. These cells were "neutral"

cells made with 10 percent amalgams. When a cell made with a particular hydrate is carried over into

the metastable range its emf wiU correspond at first to that for a supersaturated solution of the metastable hydrate and then slowly rise in value as the

7. Characteristics

7.1. Emf-Temperature Coefficient

Unsaturated standard cells have a very low emftemperature coefficient. Although it is frequently

stated that unsaturated standard cells have a dE/dt

of —0.00001 V/deg C, the emf-temperature coeffi- cient is a function of temperature, is dependent on

the concentration of the electrolyte, and increases

Percentage of Cadmium in Amalgam Figure 16. Range of temperature over which cadmium amalgams

of various cadmium content may be used in standard cells.

hydrate is converted to the more stable form. This

transition in phase is slow unless the two hydrates

are present; then the transition is relatively fast. Accordingly, overheating of normal cells may have

only transient adverse effects if the overheating is only momentary; otherwise the adverse effects may

persist for long periods of time. In any case normal

type saturated cells made with crystals of CdS04'8/3

H2O should not be heated above 43.6 °C; the upper

dotted line in figure 16 corresponds to this upper

temperature limit.

Table 11. The emfs of Weston Normal Cells made with crystals o/CdS04-8/3 H2O or crystals o/CdS04-H20*

Temperature CdSO,-8/3 H2O CdS04H20

cells cells

°C volts volts 20 1.01866 1.01333 25 1.01844 1.01417 30 1.01817 1.01501 35 1.01785 1.01587 40 1.01752 1.01672 43.4" 1.01729 1.01729 45 1.01716 1.01757 50 1.01682 1.01842

'Original data of ref. [109] were in international volts; are converted here to absolute

volts. "A better value for the transition temperature is 43.6 °C [79). of Standard Cells

as the cell ages. In table 12, values of dE/dt are given for temperatures from 15 °C to 45 °C for various weight percents of CdS04. The emf values

corresponding to cells made with the various con- centrations of CdS04 are given at 25 °C as an aid

to users in determining the emf-temperature coeffi- cient of a particular cell. For example, if a cell had

at 25 °C an emf of 1.019043 V its dE/dt would be

23
Table 12. Emf-temperature coefficients of unsaturated standard

cells

(In microvolts per degree Celsius)

CdSO, Emf, 25 'C

d£ldt

15 'C 20 °C 25 "C 30 °C 35 °C 40 °C 45 °C

wt %

41.84

42.39 42.63

42.77

42^90

volIs 1.021289

1.020019

1.019673 1.019315

1.019121

— 12.0 -3^7 -1.2 + 0.2 + 1.6

— 13,2 -5.3

-3.0 - 1.3 -0.1

— 13.5 -6.0

-3.8 -2.0

-0.7

— 12.9 -5^7 -3.6

-1.6

-0.4

— 11.5 -4^4

-2.5 -0.2 + 1.0

— 9.2 -2^2

-0.4 + 2.2 + 3.5

— 6.1 +i;o + 2.7 + 5.6 + 6.9

42.94

- 42.98 1.019043

1.018990 + 1.9 + 2.3 + 0.3 + 0.6 -0.40

0 + 0.1 + 1.3 + 1.7 + 3.8 + 4.2 + 7.3 + 7.6 43.06 43.12

43.22

1.018683 1.018704

1.018603

+ 3.2

" + 3.8

" + 4.8

+ 1.5 + 2.0 + 2.9

+ 0.8 + 1.3 + 2.2+ 0.9 + 1.6 + 2.5+ 2.5 + 3.0 + 3.9

+ 4.9 + 5.5 + 6.4+ 8.4 + 8.9 + 9.9

'43.402 ' 1.018392 -30.4 -40.6 -49.4 -56.6 -62.4 -66.6 -69.4

" — this composition corresponds to a solution saturated at 4 **C and gives a ceU with

a dEldt of zero at 25 "C.

* — supersaturated.

*^ — percentage of CdS04 in a saturated solution at 25 ^C; dEldt values given are for solutions saturated with CdSO^ • 8/3 H2O at the respective temperatures.

" -contains 0.03 N H2SO4. — 0.4 /xV/deg C at 25 °C, while if it had an emf of

1.018704 V its dEldt would be +1.3 ^iV/deg C. Inspection of the data of table 12 also shows that a

cell made with a solution containing 42.98 wt percent

of CdS04 has a zero emf-temperature coefficient at 25 °C. Vosburgh and Eppley [102] showed that this percentage corresponds closely to a solution satu- rated at 4 °C (the solutions of Vosburgh and Eppley

also contained 0.023 A'^ H2SO4). Since a solution

saturated in the range from 3 °C to 4 °C leads to cells with neghgible emf-temperature coefficients

such solutions are frequently used in the construction of unsaturated standard cells. Saturated standard cells exhibit a much larger

emf-temperature coefficient than unsaturated standare cells owing to the change in solubiUty of CdS04*

8/3H2O with temperature. They exhibit a maximum emf at 3 to 4 °C, the temperature range of the

minimum solubility of CdS04 • 8/3H2O (see table 6). Two formulas relating the emf to temperature have

been proposed. Wolff [32] working with I2V2 per- cent amalgams obtained:

E, = £20° - 0.00004060(« - 20) - 0.000000950(f - 20)^

+ 0.000000010(^-20)3 (14)

in international volts where Et is the emf at temperature t and £'20'' is the emf at 20 °C. This equation

is known as the International Temperature Formula.

It is valid from 12 to 40 °C but may be used to 0 °C

as long as the hquid and solid phases of the amalgam

are present. Vigoureux and Watts [110] using 6

and 10 percent amalgams, to extend the temperature

range, obtained:

Et = Eio' - 0.00003939(f - 20) - 0.000000903(f - 20)^

+ 0.00000000660(« - 20)^ - 0. 000000000150(« - 20)'» (15)

in international volts. These two equations are also apphcable in absolute volts. This later equation is

vahd from —20 to 40 °C; Vigoureux and Watts used

the 6 percent amalgam for the lower temperatures

but the 10 percent amalgam gives the same results as long as the amalgam contains liquid and soUd

phases; thereafter the emf decreases below that given by eq (15). The cell becomes completely

frozen at -24 °C; eq (15) gives -1139.5 /xV for £-24 »c " £^20 "c- However, when the cell becomes

completely frozen the emf is approximately 1.007 V. The emf of "neutral" saturated standard cells, as a function of temperature, based on the above

formulas, is given in figure 17. Both formulas

reproduce the maximum in emf at 3 °C to 4 °C, the

temperature range of the minimum solubility of

CdS04 • 8/3H2O. Since the acidity of the electrolyte does not appreciably affect dEjdt, the emfs of

"acid" cells would parallel the curve shown. The

displacements in the curves would depend on the

acidity employed (see sec. 6.2). In this figure the

value at 20 °C, the temperature at which international comparisons are made, is labeled Emf

standard.

The differences between eq (14) and (15) are not

large. In table 13 the differences in emf at various

temperatures from the emf at 20 °C as given by

these two formulas, are given. Since 10 percent

amalgams are now used widely in preparing satu- rated standard cells eq (15) (or 18 below) should be

used to calculate the change in their emf with

temperature.

Above 43.6 °C where CdS04 • 8/3H2O transforms

to CdS04-H20 the emf-temperature relation is given

by [109]:

£r(volts) = £43.6" + 0.000173(f - 43.6) (16)

1.01880

1,01860

1.01840

:^I.OI 820

1.01780

1.01740

^Emf s tandard

of

use 1

J

re,

limit

temperati

g

begins 1

ronsilion

> \

\ •

-20 -10 0 +10 +20 +30 +40 +50

Temperoture, C

Figure 17. Relation of the electromotive force of the cadmium

sulfate standard cell to the temperature.

24
Table 13. Differences in emf of saturated standard cells from

the value at 20 °C

Tempera- Vigoureux and Inter- Tempera- Vigoureux and International

ture Watts formula national ture Watts formula formula formula

°C microvolts microvolts °C microvolts microvolts -20 -675.6»-'' 13 229.0 234.3 -19 -575.8=-'' 14 202.3 207.4 -18 -482.0'-'' 15 173.5 178.1 — 17 — 394.2^*** 16 146 7 -16 -312.1" 17 109^9 113!l -15 -235.6" 18 75.2 77.4 -14 -164.5" 19 38.5 39.7 -13 -98.6" 20 0.0 0.0 — 12 — 37.8" 4^ 2 — 41 5 -11 + 18.2" 22 -82.2 -84!9 -10 69.3" 23 -126.1 -130.1 -9 115.8" 24 -171.6 -177.0 -8 157.9" 25 -218.8 -225.5 — 7 195.6 26 — 267.7 275 7 -6 229.2 27 -318.1 -327.4 -5 258.7 28 -370.2 -380.6 -4 284.3 29 -423.9 -435.2 -3 306.0 30 -479.2 -491.1 — 2 324.

31 526 1 548 4 -1 338.7 32 -594.6 -606.9

0 350.0 352.2 33 -654.6 -666.6 + 1 357.8 360.0 34 -716.3 -727.4

2 362.4 364.8 35 -779.6 -789.3

3 363.9 366.7 36 4 — 852 1 4 362.4 365.6 37 -911.0 -915^9

5 358.0 361.7 38 -979.1 -980.6

6 350.8 354.9 39 - 1049.0 -1046.1

7 340.8 345.4 40 -1120.6 -1112.4

8 328.3 333.3 41 (-1193.8) (-1179.3)

9 313.2 318.5 42 (-1268.9) (-1246.9)

10 295.6 301.1 43 (-1345.7) (-1315.1)

11 275.7 281.3 43.6 (- 1392.3) (- 1355.8)

12 253.5 259.0

^—for first few hours after a change from a higher temperature; electrolyte starts

to freeze at — 17 **C.

^—6 percent amalgam. —values in parenthesis were obtained by extrapolation. and unlike the cells made with CdS04-8/3 H2O the

emf-temperature coefficient is positive. The equation also apphes for the metastable monohydrate

dovfn to 20 °C. Vinal and Brickwedde used 43.4°

in eq (16) but later results [79] showed 43.6° to be a

better value and is used here.

Since 28 °C is frequently used as a maintenance

temperature for saturated standard cells, it is con- venient in such instances to have the emf-temperature relation expressed in terms of this temperature.

Equations corresponding, respectively, to eq (14)

and (15) but with 28 °C as the reference temperature

are:

Et =Eis' - 0.00005390(i - 28)

-0.00000071035(^-28)2

+ 0.000000010(f-28)3 (17)

and

Et =E2»o - 0.000052899(f - 28)

-0.00000080265(^-28)2

+ 0.000000001813(f - 28)3

-0.0000000001497(t-28)4 (18) in volts.

Table 14. Nominal emfs of saturated standard cells at some common temperatures

(Cells made with 10% cadmium amalgam)

Acidity (H2SO4) of solution in ceil

neutral ° 0.03A' 0.05^ 0.\0N

°c volts volts volts volts 20 1.018636 1.018612 1.018596 1.018556 25 1.018417 1.018393 1.018377 1.018337 28 1.018266 1.018242 1.018226 1.018186 , 30 1.018157 1.018133 1.018117 1.018077 32 1.018041 1.018017 1.018001 1.017961 35 1.017856 1.017832 1.017816 1.017776 37 1.017725 1.017701 1.017685 1.017645

" — Actually 0.00092A'; cadmium sulfate hydrolyzes to produce K2SO4 of this acidity

at 25 °C [73]. In table 14 the nominal values of the emf oi "neutral" and "acid" saturated cells are given at a

series of temperatures at which cells are most

frequently calibrated at the National Bureau of

Standards. Exact agreement with these nominal

values cannot be expected since the emfs of satu- rated cells are very sensitive to the acidity of the

electrolyte, to the amalgam composition, and to the extent of the solubihty of the glass container in the

cell electrolyte; even so they have high emf stabiUty.

Frequently, the question arises as to the effect of

small changes in temperature on the emf of satu- rated standard cells. Table 15 lists the changes in emf that are produced by changes in temperature of

only ±0.001 or ±0.01 °C. The + sign for the emf

refers to changes in emf produced by a decrease in temperature while the — sign for the emf refers

to changes in emf produced by an increase in temperature. These data show that temperature

must be controlled more accurately at the higher*

temperatures for comparable control of emf. They

also show that for precisions of 0.5 /xV the temperature must be controlled to slightly better than

±0.01 °C; for 0.05 fx\ to better than ±0.001 °C. The emf-temperature formulas given above refer

to the saturated cell as a whole, assuming that aU

parts of the cell are at the same temperature. The

separate hmbs of the cell have much larger coefficients, the negative limb having a negative coefficient and the positive limb a positive coefficient. The emf-temperature coefficient of the whole cell

is the summation of those of the two hmbs. The

emf-temperature coefficients for each hmb from 0°

to 37 °C are given in table 16; those given in parenthesis were obtained by interpolation or extrapolation of the data of Sir Frank Smith [103]. The summation of those for the two limbs are given in column 4; these agree well with those calculated

by International Temperature Formula. It is obvious, in view of these data, that saturated standard

cells must be kept at a uniform temperature for high accuracy and precision. In table 17, are given

the errors that would arise in the emf of a cell if the positive limb were at a slightly higher or slightly

lower temperature than the negative limb. Also

given are data for the case in which the temperature of the negative limb were slightly higher or lower temperature than the negative limb. It is for

25
Table 15. Effect of small changes in temperature on the emf of

saturated standard cells at various temperatures

Temperature ±0.001 °C ±0.01 °C

°C microvolt microvolt 20 + 0.04 + 0.4 25 + 0.05 + 0.5 28 + 0.05 + 0.5 30 + 0.06 + 0.6 32 + 0.06 + 0.6 35 + 0.06 + 0.6 37 + 0.07 + 0.7 + sign for the emf refers to a decrease in temperature. — sign for the emf refers to an increase in temperature. Table 16. Temperature coefficient of positive and negative limbs of saturated standard cells and of complete cells at various temperatures

Complete cell

Temper- Negative limb Positive limb International Vigoureux

ature Observed temperature and Watts

formula formula

°C voUfdeg voltjdeg voltjdeg voltldeg voltjdeg 0 -0.000288 + 0.000290 + 0.000002 + 0.000009 + 0.000009

5 -0.000297 + 0.000291 -0.000006 -0.000005 -0.000006

10 -0.000315 + 0.000295 -0.000020 -0.000019 -0.000019

15 -0.000333 + 0.000302 -0.000031 -0.000031 -0.000030

20 -0.000350 + 0.000310 - 0.000040 -0.000041 -0.000039

25 -0.000366 + 0.000314 -0.000052 -0.000050 -0.000048

28 (-0.000372)° (+0.000314) (-0.000058) -0.000054 -0.000053

30 -0.000374 + 0.000314 -0.000060 -0.000057 -0.000056

32 (-0.000376) (+0.000314) (-0.000062) -0.000059 -0.000059

35 (-0.000380) (+0.000314) (-0.000066) -0.000062 -0.000064

37 (-0.000382) (+0.000314) (-0.000068) -0.000064 -0.000067

" — Values in parenthesis obtained by interpolation or extrapolation.

this reason that copper shields are used around the

portable unsaturated cells (see sec. 5.4). Park [111]

and Eppley [112] investigated the eflfect of service

temperature conditions on the emf of unsaturated

standard ceUs and Park showed that errors in emf

arising from thermal inequalities could be reduced

to a minimum by enclosing the cell in a copper

shield. 7.2. Emf-Temperature Hysteresis

In general, if saturated or unsaturated standard

cells are subjected to slowly changing temperatures

their emf will follow closely the relations given

above. If, on the other hand, the cells are subjected to abrupt temperature changes, deviations

from the true emf will occur. These deviations are

generally referred to as hysteresis. On coohng,

the cells show first too high an emf and then a slow

decrease in emf to the equihbrium value. On

heating, the cells show first too low an emf and then

a slow rise in emf to the equihbrium values. The

magnitude of the hysteresis is given by the per- centage deviation from the equilibrium value and

is usually greater when the temperature is de- creased than when it is increased. These general

relations are illustrated in figure 18 for abrupt heat- ing of new cells from 25 to 30 °C and for abrupt

cooling from 30 to 25 °C for unsaturated standard

cells having a dE/dt of — 5 /xV/deg C in this temperature range. The magnitude of the hysteresis in each case is given by the distance marked by an "/i"

divided by the emf of the cell at the starting tem26

Table 17. Errors produced in emf of saturated standard cells if the temperature of the positive limb (or negative limb)

differed from that of the negative limb (or positive limb), by

small amounts, at various temperatures

Temperature

Temperature

difference o-c 20 °C 25 °C 28 °C 30 °C 32 °C 35 °C 37 °C

Positive limb

+ 0.01 ^v mV mV mV mV mV mV + 2.9 + 3.1 + 3.1 + 3.1 + 3.1 + 3.1 + 3.1 + 3.1 + 0.005 + 1.5 + 1.6 + 1.6 + 1.6 + 1.6 + 1.6 + 1.6 + 1.6 + 0.001 + 0.3 + 0.3 + 0.3 + 0.3 + 0.3 + 0.3 + 0.3 + 0.3 -0.001 -0.3 -0.3 -0.3 -0.3 -0.3 -0.3 -0.3 -0.3

-0.005 - 1.5 - 1.6 -1.6 - 1.6 -1.6 - 1.6 -1.6 -1.6

-0.01 -2.9 -3.1 -3.1 -3.1 -3.1 -3.1 -3.1 -3.1

Negative limb

+ 0.01 -2.9 -3.5 -3.7 -3.7 -3.7 -3.8 -3.8 -3.8 + 0.005 - 1.5 -1.8 -1.9 -1.9 -1.9 -1.9 -1.9 - 1.9 + 0.001 -0.3 -0.4 -0.4 -0.4 -0.4 -0.4 -0.4 -0.4

-0.001 + 0.3 + 0.4 + 0.4 + 0.4 + 0.4 + 0.4 + 0.4 + 0.4 -0.005 + 1.5 + 1.8 + 1.9 + 1.9 + 1.9 + 1.9 + 1.9 + 1.9 -0.01 + 2.9 + 3.5 + 3.7 + 3.7 + 3.7 + 3.8 + 3.8 + 3.8

1 1

h

1 1 1

on cooling

on heating

1 1 1 1 1 0 10 20 30 40 50

hours

Figure 18. Relations showing the emftemperature hysteresis of

unsaturated standards when abruptly heated or cooled 5 °C.

perature (this emf may be taken as unity). This

figure clearly shows that the magnitude and duration of hysteresis obtained on cooling is nearly

double that found on heating; in either case the

magnitude of the hysteresis is less than 0.01 percent.

For unsaturated cells the magnitude and duration

of hysteresis depend on the type of construction,

age, acidity and concentration of the electrolyte, on the purity of the materials, and on the rate and

magnitude of the temperature change. To these

must be added the size and change of solubility with

temperature of the crystals of CdS04 • 8/3 H2O

for the saturated cell. Therefore, it is not possible

to give quantitative data that may be apphed as
corrections to the emf of cells under diversified

conditions involving abrupt temperature changes.

Furthermore, the magnitude of hysteresis for un- saturated cells increases with age; a cell 10 years old

exhibits about 10 times the hysteresis of a new cell. In general, for a 5 °C abrupt change in temperature

the hysteresis of new unsaturated cells ranges from

0.01 to 0.02 percent (0.0001 to 0.0002 V) on coohng

and from 0.005 to 0.01 percent (0.00005 to 0.0001

V) on heating. The cells usually recover their

original emf within 1 to 2 days after cooling and

within 10 to 12 hours after heating. For older

cells, hysteresis may persist for days or even

months. Proportionate hysteresis is to be expected

for larger or smaller temperature intervals than 5

°C. In general, saturated cells show less hysteresis

than unsaturated ones. In some cases, saturated

standard ceUs do not exhibit the "overshoot"

typical of hysteresis. Instead they approach

steady emfs at a new temperature at an exceedingly

slow rate. This phenomenon here called "lag" is sometimes referred to as "negative hysteresis"

and is attributed to the slowness in the precipitation of CdS04 • 8/3 H2O on coohng or to the slowness

of dissolution of CdS04 • 8/3 H2O on heating.

Many explanations have been given for hysteresis

but no single factor is alone responsible. The

difference between the heat capacities of the two

limbs of the cell, changes in solubihty of CdS04'8/3

H2O (for the saturated ceU) with temperature, septa

(in the unsaturated cell), and the disturbances of

equilibrium conditions within the cell during temperature changes must all contribute to hysteresis. In any case standard ceUs should be maintained at constant temperature if at all possible. Temperature fluctuations may be kept at a minimum for portable unsaturated cells by placing them in temperature-lagged boxes or in Dewar flasks. A view

of a temperature-lagged box used at the National

Bureau of Standards in the testing of unsaturated

cells is shown in figure 19.

7.3. Temperature Range

The range of temperature over which standard

cells may be used is dictated by the composition of

the amalgam, the transition temperature of the

cadmium sulfate hydrates, and the freezing point of

the electrolyte. The range over which amalgams

of various cadmium contents may be safely used was shown above in figure 16. For I2V2 percent

amalgams this range is 12 to 62 °C; for 10 percent

amalgams it is —8 to 51 °C. Both of these amalgams may be used for a short time (2 to 3 hr) below

these temperatures (I2V2 percent amalgam to 0 °C;

10 percent amalgam to — 20 °C) or as long as the

amalgams consist of two phases, sohd and Hquid.

However, for work of the highest precision the cells should be confined to the temperature ranges shown

in figure 16. A 6 percent amalgam should be chosen for lower

temperatures; its useful range is — 24 to 28 °C. For

high temperatures amalgams of high cadmium conFiGURE 19. Temperature-lagged box used at the National Bureau

of Standards in the testing of unsaturated standard cells.

tent should be used, viz, a 14 percent amalgam wiU

give ceUs that could be used, from 24 to 67 °C. It may be possible to use unsaturated standard cells to temperatures of the boihng point of the electrolyte (slightly above 100 °C) by using amalgams of

high cadmium content (above 20 percent) but these

cells would not be suitable for precision work much

below 80 °C. At temperatures above 43.6 °C, even though the

higher-percentage amalgams are satisfactory, satu- rated cells must be made with crystals of CdS04-

H2O since this is the stable form of soUd cadmium

sulfate above 43.6 °C. For unsaturated standard

cells no such problem exists and attention need be

given only to the composition of the amalgam.

The lower hmit of use is — 24 °C where the cell becomes completely frozen. Freezing begins at — 17°C. CeUs completely frozen at — 24°C will behave normally after thawing if freezing has not caused a fracture in the cell. The time required

for the cell to recover its normal emf may be long,

however. The internal resistance of standard cells

at — 10 °C is about 6 times that at 25 °C and this increase in internal resistance may hmit the use of

the cell. Significant temperatures in the use of standard

cells are summarized in table 18. In practice, cells should probably be subjected to a somewhat lesser range of temperatures than shown; — 16 to 40 °C is a good range for saturated ceUs and 4 to 40 °C for unsaturated ones.

7.4. Emf-Pressure Coefficient

The effect of pressure on the emf of a galvanic

cell at constant temperature is given by

(dE\ -(M^k .

where is the volume change in cubic centi- meters at atmospheric pressure per faraday, n is

27
Table 18. Significant temperatures in the use ofstandard cells (also temperature limits for use ofstandard cells)

Temperature

Remarks

T

67 152.6 Upper limit for 14 percent amalgam, unsaturated cell 62 143.6 Upper limit for I2V2 percent amalgam, unsaturated cell 51 123.8 Upper limit for 10 percent amalgam, unsaturated cell 43.6 110.5 Transition temperature for CdS04-8/3 H2O = CdS04'H20

40 104 Upper practical limit recommended for standard cells 28-37 82.4-98.6 Suitable range for thermostated air boxes

28 82.4 Temperature at which primary standard of United States

is maintained; also upper limit for 6 percent amalgam

24 75.2 Lower Umit for 14 percent amalgam

20 68 Weston Normal Cell has a nominal emf of 1.0186360 absolute volts

12.1 53.8 Lower limit for I2V2 percent amalgam

4 39.2 Lower practical limit for unsaturated cells 3-1 37.4-39.2 Emf of Weston Normal Cell is at a maximum; solubility of CdS04-8/3 H2O is a minimum

-8 17.6 Lower limit for 10 percent amalgam

-17 1.4 Freezing begins

-24 -11.2 Cell completely frozen; will recover normal emf in about

a week after thawing, if not cracked

the number of equivalents involved in the reaction,

F is the faraday, and A: is a conversion factor,

0.101325, which converts cubic centimeter-atmospheres into joules, i.e., volt coulombs. The

volume change may be calculated from the atomic

or molecular weights and the densities of the reactants and products of the reaction; for saturated

standard cells this reaction is given by eq (7). At

20 °C, the accepted densities of Cd, Hg2S04,

CdS04-8/3 H2O, Hg, and a saturated solution of

CdS04 are, respectively, 8.648, 7.56, and 3.090 g cm -3 and 13.5463 and 1.6119 g ml'i [113]. Using

these data, accepted atomic weights on the C*^

scale [114] and 96,487 coulombs/gram-equivalent

[115] for the faraday equation 19 gives 6.4 /nV/atm

for {dEldP)T. This value agrees excellently with

6.1 /xV/atm found experimently by Ramsey [116]

(this value is an interpolated value, Ramsey found

6.02 /iV/atm at 20.4 °C and 7.6 ^tV/atm at 19 °C).

The effect of pressure on an unsaturated cell would be nearly the same, since unsaturated cells are made with solutions that are nearly saturated.

A priori no effect on the emf of sealed cells would be expected providing the pressure was

insufficient to fracture the cell. Recent experiments by Catherine Law and D. N. Craig of this Bureau, in which the external pressure on the cell was increased to approximately 5 atm, has con- firmed this. Standard cells have an air space

above the electrolyte which would act as a cushion

to absorb pressure change even if the elasticity of

the glass container were such as to transmit pres- sure change to the cell components.

In the original paper [115] values based on the physical and chemical scales of atomic weights were given. The value given here is based on an atomic weight of

silver relative to the C'^ scale of atomic weights, namely, 107.870.

7.5. Internal Resistance

The internal resistance in the vicinity of 25 °C

ranges from 100 to 500 fl for unsaturated cells and

from 500 to 1000 O for saturated cells manufactured

in the United States. The internal resistance

increases at the rate of about 2 percent for a decrease in temperature of one degree C; this fact should be taken into account in calculating the IR

drop in a cell (see table 19). The resistances of

the positive and negative limbs of symmetrical

H-shaped cells are approximately equal. As a cell ages its internal resistance increases slightly. The

internal resistance of a standard cell may be

estimated by momentarily placing a 10-Mfl resistor across the cell terminals and reading the emf, Er. The value of the internal resistance is then given by

R = W{E^ - Er)IEr ohms, (20)

where Eq is the open-circuit emf. The cell recovers

its initial emf within a few minutes after the 10-Mfl

resistor is removed. If a cell exhibits high internal

resistance or insensitivity it may contain a gas

bubble at the anode which may be frequently re- moved by tapping the cell when inclined 45°.

If this treatment is ineffective a new cell is recommended.

7.6. Effect of Current

Standard cells are not intended to serve as sources of electric current. Even so, the question

frequently arises about the effect of current on

standard cells, especially the unsaturated type.

When standard cells are charged or discharged the

28
Table 19. Effect of current on the emf of unsaturated standard cells at 25 °C

Current, Internal resistance (initial change) ampere

100 ohms 500 ohms

t^y

10-10 0.01 0.05

io-» 0.1 0.5 io-» 1.0 5.0 10-' 10.0 50.0

10-^ 100.0 500.0

10-= 1000.0 5000.0

Changes owing to changes in amalgam iVa) and electrolyte

{Ve) composition (after one year)

Va

I^V

^ nv10-10 10-5

10 -« 0.03

10-' oos' 0.26

io-« 0.5 002 2.6 10-' 0.05 5.3 0.17 26.3

Changes arising from electrode polarization (after one year)

m''

JO -10 0.1 0.3 io-» 0.7 2.0

10 -» 4.3 12.5 10-' 26. 76. 10-= 159. 466. 10-= 970. 2844.

" — dash means that the change is less than 0.01 jiV.

rise or drop in emf is initially dictated by the IR

drop and subsequently by a voltage change, Vc, associated with the chemical changes in the cell, and by electrode polarization and changes in internal

resistance. For the unsaturated cell Vc is made up

of two parts: Va, the change in emf associated with

the change in the composition of the amedgam during

charge or discharge and Ve, the change in emf

associated with the change in electrolyte content of

the solution during charge or discharge. When

unsaturated cells have been discharged until their

solutions are saturated the latter no longer apphes.

For saturated cell, Vc consists of Va only.

The magnitude of Va obtained on discharge at 25 °C is given by ^ ,. . , -0.001067/f

Kq (in volts) = = —1.106X10 ^

(21)

where / is the current in amperes, t is time in seconds, W is the total weight of the amalgam in grams, and F is the faraday (96487.0 coulombs).

For NBS saturated cells the total weight of amalgam

is generally about 10 g. In this case eq (21) reduces

to Fa (in volts) =-1.106 X 10-9 (22)

For unsaturated cells having internal resistances of

100 and 500 fl the weight of the amalgam is approximately 67 and 20 g, respectively.

The magnitude of Ve obtained on discharge at 25 °C is given by

Ve (in volts)

0.161 It FW 1.67 X 10-It W

(23)

where W is now the weight of the electrolytic solu- tion in grams and the other symbols have the meaning given above. For unsaturated cells the weight

of solution is about 100 and 20 g, respectively, for

cells having internal resistances of 100 and 500 H.

Equation (23) then becomes

Ve (in vohs) = - 1 .67 X 10- 8 (24)

for the 100-fl cell and

Fe(in volts) =- 8.35 X 10-8 (25)

for the 500-n cell. The magnitudes of electrode polarization during

discharge may be calculated, within a few microvolts, by the equation

log AJ? (in microvolts) = 1.6048 + 0.786 log

(26)

where A£' is the change in emf, in microvolts,

arising from electrode polarization and changes in internal resistance, C is quantity of electricity in coulombs (ampere seconds) and A is apparent

(geometric) electrode area in square centimeters

[117]. For unsaturated cells having internal

resistances of 100 and 500 fi, A is about 5.5 cm^ and

1.4 cm^, respectively. Some miniature types having

internal resistances of about 1000 H and apparent

surface areas of less than 1 cm^ are also available.

The length of time standard cells will sustain a discharge depends on the amount of material in the

cells; most unsaturated cells of United States manufacture contain sufficient material to yield

from 700 to 5000 coulombs if the current is kept

below about 2 X 10"^ A per cm^ of electrode area. When an external load is removed, the cells re- cover their initial emf, provided the discharge has

not been a prolonged one. The time required for recovery depends on the severity of the discharge.

For example, if an unsaturated cell (internal re- sistance = 100 H) is discharged for 5 min at 6 X 10"^

A cm-2 it will recover its original emf within 5 fxY

in 30 min and will completely recover after 6 hr. At a higher current density of 6 X 10"^ A cm"^ the

emf will be about 180 /aV below its original value

after 30 min, 5 fjN after 6 hr, and several days will be required for complete recovery. If the cell were discharged at a current density of 6 X 10-^ A

cm-^ to a low cutoff voltage of 0.001 V, recovery wiU

be exceedingly slow requiring several months and

'= Includes any change in internal resistance that may occur as a cell is discharged.

29
full recovery will not be attained after this prolonged

period because the normal emf will have declined

(see below). The time of recovery, therefore, is seen to depend on the rate and extent of the

discharge.

In table 19 changes in emf caused by various cur- rents of low magnitude are illustrated for cells having electrode cross-sectional areas of 5.5 cm^ and

1.4 cm^ and internal resistances of 100 and 500 fl. The changes arising from internal resistance are

initial changes whereas those arising from changes

in amalgam composition, Va, electrolyte composition, Ve, and electrode polarization (see footnote 16), A£, are functions of time of discharge (for illustra- tion, a period of 1 year is chosen for these). The

total changes in emf during a discharge is the sum

of the four effects. On a current density basis it should be noted the changes for the lOO-O and 500-

Cl cells are nearly identical since the electrode area

for the former cell is approximately five times that of the latter. Standard cells may be short-circuited momentarily

without permanent damage to the cells. The cells

will recover their original emf within a few minutes

after taken off short circuit. If kept on short

circuit they wiU be completely discharged within

V2 to 2 days depending on the size and internal

resistance of the cell, and wiU not recover their

initial emf. The short circuit current is given by

the ratio of the open-circuit emf and the internal

resistance of the cell. For cells having internal

resistances of 100 Cl and 500 fl, the short-circuit

(flash) current will, therefore, be 1x10"^ and

2 X 10-^ A, respectively.

7.7. Effect of Light

Mercurous sulfate is sensitive to light and changes

in color at a slow rate through tan, to gray-brown,

to dark brown, and finally to black. Although

standard cells having discolored mercurous sulfate may have normal emfs [118] they exhibit slower

approach to equilibrium values after temperature or other changes. Standard cells should, therefore,

be mounted in nontransparent cases or kept in the

dark and used only for short periods at a time under

diffuse light.

7.8. Effect of Shock

Mechanical shocks insufficient to fracture or

break or scramble the components of unsaturated

standard cells have no lasting effects on the cells. Unsaturated standard cells packaged in excelsior

and shipped by common carrier to the National

Bureau of Standards have been observed to perform

satisfactorily. When subjected to shocks of 10 to 40 g for durations of 6 to 18 msec unsaturated cells exhibit large transient changes in emf ranging from

4,200 to 31,000 AtV [119]. After the shock the ceUs

immediately recover their original emf within 2 ^tV. The transient emfs observed during shock probably

arise from a disturbance of the mercury and

amalgam surfaces during the period of shock.

On the other hand, the usual types of saturated

cells of United States manufacture should not be

subjected to sudden shock, should not be shipped

by common carrier, should be transferred by

messenger, and should not be tilted more than 45°. Some new saturated standard cells of novel design

are stated to be portable, i.e., may be shipped by

common carrier. However, studies over a period

of time wiU be required to ascertain the long-term

stability of their emf.

7.9. Effect of Vibration

Vibrations at frequencies from 10 to 1,000

Hz (c/s) with accelerations of 1 to 10 g have no

lasting effects on the emf of unsaturated standard

ceUs [119]. During the vibration, however, rather

large a-c voltages of the same frequency are generated. For an unsaturated cell having an in- ternal resistance of 500 CL this a-c voltage ranges

from about 25 /u-V at 1 g and 1000 Hz to 9900 /aV

at 10 g and 50 Hz. Furthermore, there is a decrease

in the d-c emf ranging from 3 /liV at 1 g and 1000 Hz

to about 200 fxY at 10 g and 100 Hz. In general at frequencies above 100 Hz the waveform of the a-c voltage is sinusoidal whereas below 100 Hz it is nonsinusoidal owing to the resonance of the various

components of the cell. In most cases the a-c and

d-c effects of the vibration appear and disappear

instantaneously when vibration is started or stopped.

In some instances the d-c change may be rapid in the initial moments of vibration and then build

up shghtly in an exponential manner for 2 or 3 min.

In these instances when the vibration is stopped the

d-c emf decays in the same fashion as it was built up.

8. Life of Standard Cells

Saturated standard cells have an exceedingly long

life. Some cells at the National Bureau of Standards have been in use for nearly 60 years, and they

have retained their emfs within a few microvolts; see section 2.3. On the other hand, unsaturated

cells at room temperature (about 25 °C) decrease in emf at a rate of about 20 to 40 ^tV per year [48]. This decrease in emf is equivalent to a corrosion

rate for the amalgam of about (6.8 to 13.6) X 10~^ A

cm~^. Since the emf of new unsaturated standard

cells generally range from 1.01900 to 1.01940 V,

depending on the concentration and acidity of the

electrolyte, these cells on the average reach an emf of 1.01830 V within 23 to 37 years, providing

they are maintained at 25 °C or thereabouts, and are not subjected to abuse, such as discharging or charging current; a practical fife time for these ceUs

is probably 12 to 18 years. When unsaturated

standard cells reach an emf of 1.01830 V or lower

the cells generaUy behave erraticaUy (largely be- cause the electrolyte may become supersaturated on cooUng), have large emf-temperature coeffi- cients, and show excessive emf-temperature hysteresis. The life of the ceU is considerably reduced

30
if the cell is stored at higher temperatures. The

rate of decrease in emf is approximately doubled

for every 12 °C increase in temperature [120]: thus

at 37 °C the life of an unsaturated cell would be one

half that given above. Cells of the miniature type

with short diffusion path between electrodes wiU

have a shorter hfe.

9. Refe

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[5] Ref. [3]. First Report, p. 9. Cambridge, 1862.

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[33] Announcement of a change in the value ot the International Volt, NBS Circ. 29, p. 9, footnote 1, Dec. 31, 1910: records of NBS carried results of emf of saturated stand- ard cells to 6 decimals from 1911 until 1934 and to 7 decimals since then.

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31
oxide in saturated standard cells, J. Res. NBS 27,

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32
[88]

[89]

[90]

[91]

[92]

[93]

[94]

[95]

[96]

[97]

[98]

[99]

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[101]

[102]

[103]

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cells and its relation to change of electromotive force due to pressure, Phys. Rev. XVI, No. 2, 105 (1902). [117] Based on measurements made at Natl. Bur. Standards by Phyllis R. Robb and W. J. Hamer, 1950-52; see also

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shock on unsaturated standard cells, J. Res. NBS 66C

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[123] F. X. Lamb. Aging of standard ceUs, NBS Circ. 524, Electrochemical constants, Ch. 11, 1953. 10. Appendix 1

U.S. Law of 1894, 53d Congress, 28 Stat., Ch. 131, p. 102 (Public-No. 105)

An Act To define and establish

Be it enacted by the Senate and House of Representatives of the United States of America in

Congress assembled. That from and after the pass- age of this Act the legal units of electrical measure

in the United States shall be as follows:

First. The unit of resistance shaU be what is known as the international ohm, which is substantially equal to one thousand million units of

resistance of the centimeter-gram-second system

of electro-magnetic units, and is represented by

the resistance offered to an unvarying electric cur- rent by a column of mercury at the temperature of

melting ice fourteen and four thousand five hundred

and twenty-one ten-thousandths grams in mass, of

a constant cross-sectional area, and of the length

the units of electrical measure

of one hundred and six and three tenths centi- meters. Second. The unit of current shall be what is known as the international ampere, which is one- tenth of the unit of current of the centimeter-gramsecond system of electro-magnetic units, and is the practical equivalent of the unvarying current,

which, when passed through a solution of nitrate of silver in water in accordance with standard

specifications, deposits silver at the rate of one thousand one hundred and eighteen millionths of a gram per second.

Third. The unit of electro-motive force shall be

what is known as the international volt, which is the electro-motive force that, steadily applied to a

33
conductor whose resistance is one international

ohm, will produce a current of an international

ampere, and is practically equivalent to one thousand fourteen hundred and thirty-fourths of the

electro-motive force between the poles or electrodes

of the voltaic cell known as Clark's cell, at a temperature of fifteen degrees centigrade, and prepared

in the manner described in the standard

specifications.

Fourth. The unit of quantity shall be what is known as the international coulomb, which is the

quantity of electricity transferred by a current of one international ampere in one second.

Fifth. The unit of capacity shall be what is known as the international farad, which is the ca- pacity of a condenser charged to a potential of one

international volt by one international coulomb of

electricity.

Sixth. The unit of work shall be the Joule, which

is equal to ten million units of work in the cen- timeter-gram-second system, and which is practically equivalent to the energy expended in one second by an international ampere in an international ohm.

Seventh. The unit of power shall be the Watt,

which is equal to ten million units of power in the

centimeter-gram-second system, and which is practically equivalent to the work done at the rate of one Joule per second.

Eighth. The unit of induction shall be the Henry,

which is the induction in a circuit when the electro- motive force induced in this circuit is one inter- national volt while the inducing current varies at the rate of one Ampere per second.

Sec. 2. That it shall be the duty of the National

Academy of Sciences to prescribe and publish, as soon as possible after the passage of this Act, such

specifications of details as shall be necessary for the practical application of the definitions of the

ampere and volt hereinbefore given, and such specifications shall be the standard specifications herein

mentioned.

11. Appendix 2

Public Law 617 — 81st Congress

(Chapter 484 -2d Session)

(S. 441)

AN ACT

To redefine the units and establish the standards

of electrical and photometric measurements.

Be it enacted by the Senate and House of Representatives of the United States of America in

Congress assembled. That from and after the date

this Act is approved, the legal units of electrical and photometric measurements in the United States

of America shall be those defined and established

as provided in the following sections.

SEC. 2. The unit of electrical resistance shall be the ohm, which is equal to one thousand million

units of resistance of the centimeter-gram-second

system of electromagnetic units. SEC. 3. The unit of electric current shall be

the ampere, which is one-tenth of the unit of current

of the centimeter-gram-second system of electro- magnetic units. SEC. 4. The unit of electromotive force and of

electric potential shall be the volt, which is the

electromotive force that, steadily applied to a con- ductor whose resistance is one ohm, will produce a current of one ampere.

SEC. 5. The unit of electric quantity shall be

the coulomb, which is the quantity of electricity

transferred by a current of one ampere in one

second.

SEC. 6. The unit of electrical capacitance shall be the farad, which is the capacitance of a capacitor that is charged to a potential of one volt by one coulomb of electricity.

SEC. 7. The unit of electrical inductance shall be the henry, which is the inductance in a circuit such that an electromotive force of one volt is

induced in the circuit by variation of an inducing

current at the rate of one ampere per second.

SEC. 8. The unit of power shall be the watt,

which is equal to ten million units of power in the

centimeter-gram-second system, and which is the

power required to cause an unvarying current of one ampere to flow between points differing in potential by one volt. SEC. 9. The units of energy shall be (a) the

joule, which is equivalent to the energy supplied

by a power of one watt operating for one second,

and (b) the kilowatt-hour, which is equivalent to the energy supplied by a power of one thousand

watts operating for one hour.

SEC. 10. The unit of intensity of light shall be

the candle, which is one-sixtieth of the intensity

of one square centimeter of a perfect radiator,

known as a "black body", when operated at the

temperature of freezing platinum.

SEC. 11. The unit of flux of fight shall be the

lumen, which is the flux in a unit of solid angle

from a source of which the intensity is one candle.

SEC. 12. It shaU be the duty of the Secretary of

Commerce to establish the values of the primary

electric and photometric units in absolute measure,

and the legal values for these units shall be those

represented by, or derived from, national reference

standards maintained by the Department of

Commerce.

SEC. 13. The Act of July 12, 1894 (Public Law

Numbered 105, Fifty-third Congress), entitled "An

Act to define and estabhsh the units of electrical measure", is hereby repefded.

APPROVED July 21, 1950.

34
12. Appendix 3

Standard Cells with Deuterium Oxide

The electromotive forces of saturated standard

cells, made with deuterium oxide and normal water

follow:

Percentage of D2O in the water

0.02 ^ 1020

30

40

5060

7080

90

100 (extrapolated from

98 percent)

Emf at 20 "C," volts 1.018603

1.018567

1.018531 1.018495

1.018459

1.018423

1.018384

1.018344

1.018301 1.018255 1.018204

a — Normal water contains 0.02 percent deuterium oxide.

* — Original data were reported in international voLts and have here been

converted to absolute volts.

These cells were made with crystals of cadmium sulfate in equilibrium with the respective solutions. This was accomplished by saturating each solution with anhydrous cadmium

sulfate; the crystals that separated from the solution were then

in equihbrium with the saturated solution. The solutions were

then made 0.035 to 0.042 N with respect to sulfuric acid; the above emfs are reported on a uniform basis of 0.04 A'. Cadmium sulfate is about 8 percent less soluble in heavy water

than in normal water in the temperature range of 0 to 60 °C. This comparison follows:

Temperature 'C

Moles CdS04 per mole water

normal water ^

0 0.06546 0.06026

5 .06555 .06033

10 06566 .06042

15 .06580 06055 CdS04 8/3 aq " 20 .06600 .06073 25 .06627 .06097 30 .06663 !06130 35 .06710 .06174 40 .06773 .06232 45 .06857 .06311 50 .06955 .06415

45 .06797 .06326 50 .06659 .06181 CdS04 aq" 55 .06522 .06037 60 .06385 .05893

" — here aq means either H2O or D20. "—contains 0.02 percent heavy water. Studies of the changes in the difference between the emf of

cells made with normal and heavy water with time have given

supplemental evidence on the stabihty of the National Reference Group of Standard Cells. These studies have shown that this difference is within the spread, namely, 0.05 /xV/year, discussed under item (1) on maintenance in section 2.3. However, cells made with heavy water have not, to date, shown the same

internal consistency as those made with normal water, i.e., individual cells show greater day-to-day variations in emf than normal water cells. 13. Appendix 4

Coulomeler {Smith form) measurements* by the United States in the Physikalisch-Technische Reichsanstall at Charlottenburg, 1931

Experiment Number**

I 2 3 4 5 7 8 9 10

A 4.09189 4.09328 4.09600 4.08556 4.08615 4.10120 4.09495 4.09105 4.16814 B 4.09172 4.09317 4.09591 4.08543 4.08621 4.10123 4.09.507 4.09110 4.16824 C 7230.00 7199.85 7200.06 7199.84 7200.01 7200.16 7200.06 7199.84 7.320.05 D 1.999989 1.999973 1.999973 1.999975 1.999973 1.999971 1.999975 1.999981 1.999985 E 0.000005 0.000005 0.000005 0.000005 0.000005 0.000005 0.000005 0.000005 0.000005 F 1.018301 1.018429 1.018411 1.018336 1.018334 1.018331 1.018328 1.018332 1.018334 G 1.018.301 1.018430 1.018402 1.018336 1.018334 1.018330 1.018328 1.018330 1.018.332 H 1.012582 1.017114 1.018145 1.015176 1.015326 1.019020 1.017458 1.016712 1.018796

J 1.018259 1.018390 1.018694 1.018296 1.018293 1.018288 1.018283 1.018283 1.018286 K 0.000011 0.000011 0.000014 0.000014 0.000013 0.000015 0.000013 0.000013 0.000012 L 1.018116 1.018299 1.018234 1.018229 1.018220 1.018238 1.018266 1.018070 1.018144 ave = 1.018202 M 1.018187 1.018370 1.018305 1.018300 1.018291 1.018309 1.018337 1.018141 1.018215 ave = 1.018273

•Electrical measurements made in PTR units; (USA — PTR) ohm = 9 ppm; (USA — PTR) volts =--61 ppm.

**Experiment Number 6 was conducted with a porous-cup coulomeler, therefore, not included. A — weight of silver deposit in one coulometer, grams. B — weight of silver deposit in second coulometer in series with A, grams. C — time of electrolysis, seconds. D — resistance R of standard resistor, ohms. E — correction for resistance owing to leads, etc., ohm.

F — average emf of control cell at 20 "C, volts. G — emf of control cell at 20 *C at end of run, volts. H — //? in standard resistor, volts; / is current in amperes.

J — emf of German reference cell at 20 *C, volts. K — correction to J to give mean value of Kleiner Stamm (primary volt standard of Germany), volt. L — German vaUie of selected cell at 20 "C; given by

olts 1.0011180/' M — USA (USA units) value of selected cell at 20 "C; given by M = 1.000070Z, volts, since (USA - PTR) ohm = 9 ppm and (USA - PTR) volts = — 61 ppm

35
14. Appendix 5

Constant-Temperature Oil Baths

Constant-temperature oil baths of the type used

to maintain the National Reference Group of Standard Cells at a constant temperature were shown in figure 2. In this appendix a general description of a

typical bath is presented. A circular bath of stain- less steel or nickel-plated copper, about 75 cm in diameter and 45 cm deep, may be used (all other

metal parts are also of steel or nickel-plated copper

to prevent the possibility of galvanic corrosion). It

is fiUed with oil (see Appendix 6) to a depth of about

35—40 cm. The oil is stirred by a centrally located

two-blade agitator which produces an upward and

rotary circulation. The agitator (9) is about 48

cm in length, 5 cm wide, 0.3 cm thick, has a pitch

of 20° and rotates at about 45 rpm. It is supported on a shaft (1) which is supported in ball bearing

housings at the bottom and top of the bath (the one

at the top is supported in a metal slab (3), about 10

cm wide and 0.3 cm thick, which rests on the central

top edges of the bath. A drip cup (4) is placed on

the shaft at the position shown to collect any grease

that may flow down the shaft from the ball bearing

housing. To guide the direction of oil circulation a metal baffle (5), 25 cm in height, is placed in the

bath in the position shown in figure Al. This baffle extends to 10 -cm of the bottom of the bath and

shghtly above the agitator. This baffle serves

also to protect the cells from direct influence of

the heating element and the cooling coils. Heating is suppHed by a heating coil (8) of bare

manganin wire wound on a Pyrex rod having a diameter of 1 cm. This rod is supported about 5

cm above the bottom of the bath by glass supports

and circles the bath; its position is clearly shown

in figure Al. The heating coil has a resistance of

90 ohms and operates on 110-V d-c supply. A

bank of four incandescent lamps in parallel is included in the heating circuit; two lamps operate

intermittently during control while two operate

continuously, i.e., some heating is provided continuously and some intermittently. Cooling coils

(7), (11), as shown, are also provided. Through

these coils flow oil of the same type as used in the bath. These coils are connected to similar

coils in the freezing compartment of a refrigerator

and the required coohng is attained by controlling

the rate of oil flow. Cooling by this means is re- quired only in emergencies or when tests of standard cells are made at temperatures below room

temperature. The temperature of the bath is determined by a platinum resistance thermometer

and a Mueller bridge; the thermometer is supported

at the position (2) shown in the figure. The lamps

in the circuit also aid an operator in determining

when the bath has attained a peak temperature

(one reading on Pt resistance thermometer) and a

Figure Al. Cross-sectional sketch of a constant-temperature oil bath used at the National Bureau of Standards.

1 — Agitator shaft 7 - Cooling coUs

2 — Thermometer mount 8 — Healer

3 — Center plate 9 — Agitator 4— Drip cup 10 — Mercury-toluene regulator 5 — Baffle 11 — Cooling coil connector

6 — Cell rack 12 — FenwaU thermoswitch

36
lower temperature (second reading on Pt resistance

thermometer) and thereby determine the mean

temperature of the bath (the period between peak

and low temperatures is about 15 sec).

A mercury-toluene regulator (10) is used as the

temperature-control element. It consists of a

reservoir in the form of a rectangle, 40 cm on an

edge, made of thin-waUed Pyrex tubing, 1.5 cm in diameter, containing the thermometric substance

(toluene), the volume- changes being transmitted by a U-shaped connecting tube filled with mercury to a

vertical capillary as shown in the figure. The U- shaped connecting tube is provided with a downward extension from the toluene reservoir to allow

for expansion and contraction of the toluene thereby

preventing the toluene from reaching the capillary.

The capillary is about 1 mm in diameter and 8 cm

long and the adjustable electrical contact to the

mercury is made with a steel piano wire. The

permanent electrical terminal to the mercury is

made through a seal about 4 cm below the capillary tube. This terminal is connected to the electro- magnet of a sensitive 150-fi relay, whereas the

adjustable terminal is connected to one terminal of a long-life Lalande battery of 2.5 volts; the other

terminal of this battery is connected to the elec- tromagnet. The secondary of the relay is in the

110-voh d-c heater circuit. The regulator is sup- ported on the baffle about 10 cm from the bottom of the bath. The capillary extends above the oil and

is therefore, exposed to the air; this exposure, however, does not cause a significant error in regulation.

A Fenwall thermoswitch (12) is included in the heat- ing circuit as a precaution in case the regulator does

not function; tfiis thermoswitch opens the heating

circuit at a preset value, usually 1 °C, above the

temperature at which the bath is controlled. The

standard cells are mounted in the bath on the metal

rack (6) in the manner described in the text (sec. 2.3). Other details on the bath are shown in figure Al.

15. Appendix 6

Oil for Constant-Temperature Baths

The quahty of the oil in which standard cells are submerged is of considerable importance, and for

this purpose a white mineral oil with the following

specifications is used:

Colorless, odorless, tasteless; Viscosity, in poises at 25 °C, 0.245*;

Specific gravity, 25 °C, 0.846; Hashpoint, 171 °C (340 °F); Burning point, 207 °C (405 °F);

Acidity (mg KOH/gm oil), none;

Sulfur, none; Oxidation number, 0; Gum, on heating in oxygen, none;

Discoloration, on heating in oxygen, slight orange.

*Sufficiently low as to make the stirring effective. In addition this oil, when new, has a resistivity

of about 3 X 10" ohm-cm or higher.

16. Appendix 7

Constant-Temperature Enclosure for

Saturated Standard Cells

The MueUer-Stimson temperature-control box

for saturated standard cells was described in general terms in section 5.4. In this appendix more

details are given. In this control box, the temperature of an outer aluminum case is automatically

controlled by a conventional mercury-in-glass

thermoregulator. A second aluminum case, inside

the first and thermally well insulated from it, contains five saturated standard cells. The inner case assumes a temperature very nearly the time

average of that of the outer case. The inner case

is 3^/8 in. long, 2^U in. wide, and 4 in. deep inside. This case is a casting with V4-in. walls and

in. bottom. A V4-in. -thick cover is secured to the

casting by numerous screws to improve thermal

contact. The inner case is spaced within the outer one with V2 in. balsa wood on all six sides. The

outer case is also a casting, with ^/s in. sidewalls

and V2 in. bottom. A cover of sheet %-in. aluminum is secured to the top by screws. The endwaUs

of the casting are plane on the inside and cylindrical on the outside, the thickness at the edges

being ^1% in. and in. at the middle; this extra thickness at the middle provides space for a vertical hole, ^^/32 in. in diameter and 6 in. deep, to ac- commodate the thermoregulator at one end and a smaller hole to accommodate a thermometer at the other end.

The outer case is insulated with a layer of balsa

wood 1 in. thick on the sides and bottom and 2 in. thick on the top. The pieces of balsa wood fit neatly into an outer wooden box. This box is of

sufficient size to provide a compartment at one end

to house a transformer, relay, and binding posts

for the necessary connections. These accessories

are mounted on a wooden panel which slides in vertical grooves in the sides of the box. The out- side dimensions of the box are 8 by 13 in., by 10

in. deep, and its weight, complete, is 22 lb. A

horizontal section of the control box is shown in figure A2.

37
Figure A2. Horizontal sketch of Mueller-Stimson temperaturecontrolled box.

The standard cells are mounted on balsa wood of

appropriate size. No. 28 (0.013 in.) insulated cop- per wire, 4 ft. in length, is used for connections to the cell terminals. About one-third of this length

is formed in a helix which is kept inside the inner case and another one-third is placed between the

inner and outer cases. The wires are brought

through the cases in smoothed saw cuts in the upper

edges of the sides and insulated additionally with

silk and glyptal whereby good thermal contact with the metal cases is assured. These thermal

tiedowns and lengths of wire reduce heat conduction to a negligible amount. Sheets of mica are placed on the surfaces of the aluminum cases, adjacent to the connecting wires, to protect them

against accidental electrical grounding. Outside

the outer case the wires are brought to binding

posts supported on a hard-rubber strip; the positive

terminal is placed on one side of the box, the negative on the other. The thermoregulator is an adjustable mercury-inglass type with a bulb 4 in. long about ^^/32 in. in diameter, and 8 in. long. It is covered with grease

to assure good thermal contact to the reservoir

into which it fits. The heating resistor. No. 38 (0.004 in.) constantan

wire, has a resistance of about 70 O and is wound

in four turns on the sides and ends of the outer aluminum case on silk fabric. One turn is placed near the top edge and another near the bottom edge

of the case. It operates on 20 V. A small, quickacting 12-V a-c relay operating on about 0.05 A is used. Power for the heater and relay is supplied

by a bell-ringing transformer rated at 50 W.

38

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