IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-21, NO. 4, NOVEMBER 1972

international value of 2e/h for use by all those laboratoriesthat wish to maintain their units of voltage via the Josephsoneffect.4) There is a definite need for developing improved voltage

reference and transport standards in order to take full advan-tage of the inherent accuracy of the 2e/h measurements. Inour opinion, difficulties alluded to above are due to the in-adequacies of these voltage standards. Determinations of2e/h are clearly pushing the existing technology in this area.

ACKNOWLEDGMENTThe results reported here were made possible only by the

dedicated efforts of a large number of individuals at the parti-cipating laboratories. At the risk of overlooking someone, weshould like to gratefully acknowledge the collaboration of thefollowing persons: J. Terrien, Director, BIPM, under whoseauspices the transfers were carried out; G. Leclerc, BIPM;J. J. Denton, J. C. Gallop, and B. W. Petley, NPL; A. F. Dunnand G. H. Wood, NRC; R. C. Richardson, I. K. Harvey, J. C.Macfarlane, and R. B. Frenkel, NSL; W. Hetzel, V. Kose,F. Melchert, and H. Fack, PTB; and B. F. Field, T. F. Finnegan,J. Toots, and T. J. Witt, NBS. We are particularly grateful toH. H. Ellis, NBS, for making the hundreds of required measure-ments on the transport standards. We should also like to thank

Miss Thelma Gable of Pan American Airways for her assistancein arranging for the shipment of the standards.

REFERENCES

[1] G. Leclerc, "Rapport sur la 12eme comparaison des etalons na-tionaux de force electromotrice," Bur. Int. Poids Mes., Sevres,France, 1970.

[2] W. G. Eicke, Jr., and J. M. Cameron, "Designs for the surveillanceof a small group of standard cells," NBS Tech. Note 430, 1967.

[3] T. F. Finnegan, T. J. Witt, B. F. Field, and J. Toots, "Measure-ments of 2e/h via the ac Josephson effect," in Proc. 4th Int.Conf Atomic Masses and Fundamental Constants, J. H. Sandersand A. H. Wapstra, Eds., to be published.

[4] J. C. Gallop and B. W. Petley, "Recent NPL work on 2e/h," inProc. 4th Int. Conf. Atomic Masses and Fundamental Constants,J. H. Sanders and A. H. Wapstra, Eds., to be published.

[5] B. W. Petley and J. C. Gallop, "Measurement of 2e/h by theJosephson effect," in Precision Measurements and FundamentalConstants, D. N. Langenberg and B. N. Taylor, Eds., NBS Spec.Publ. 343, U.S. GPO, Washington, D.C., pp. 227-229, 1971.

[6] I. K. Harvey, J. C. Macfarlane, and R. B. Frenkel, "Determinationof 2e/h based on the ac Josephson effect," Phys. Rev. Lett., vol.25, p,p. 853-856, 28 Sept. 1970.

[7] -, 'Monitoring the NSL standard of emf using the ac Josephsoneffect," Metrologia, to be published.

[8] V. Kose, F. Melchert, H. Fack, and H.-J. Schrader, "Die Bestim-mung von elh mit hilfe des Josephson-effektes," PTB Mitt.,vol. 81, pp. 8-10, Jan. 1971.

[9] V. Kose, PTB, Germany, private communication.[10] B. F. Field, NBS, Washington, D.C., private communication.[11] B. W. Petley, NPL, United Kingdom, private communication.[12] I. K. Harvey, NSL, Australia, private communication.

Standard Cells With Cd Pb Amalgam ElectrodeHIROYUKI HIRAYAMA AND KAZUMI SHIMAZAKI

Abstract-In order to improve the characteristics of standard cells,those containing composite amalgam electrode have been studied. Thecriterion and test procedures established for selecting high-qualitystandard cells have been applied. The addition of Pb to Cd amalgamhas the effect of reducing the temperature coefficient. The mostappropriate composition of amalgam is that of 10 wt % Cd, 3-9 wt %Pb, and the balance Hg. The cells with Cd-Pb amalgam electrode havean EMF higher by about 1000 gV than that of normal Weston cell, anda temperature coefficient at 20°C or -17 ,uV/°C. The other character-istics and stability are the same as those of normal cells and they are

suitable for high-precision work. Although zero temperature coefficientmay be obtained by adding Pb and Bi to Cd amalgam, its characteristicsare inferior to those of normal cells.

INTRODUCTION

N ORDER to meet the recent demand of high precision for

a voltage standard, and because of the international recon-sideration of the specification of standard cells, the Electro-

Manuscript received June 9, 1972; revised July 21, 1972.The authors are with the Tanashi Branch, Electrotechnical Labora-

tory, Tokyo, Japan.

technical Laboratory has performed various research on thissubject for about 10 years. Rules for selecting good cells,namely, criteria and test procedures, have been established and,applying them, the selection of and research in standard cellsare now being performed. Although the application of theJosephson effect as a voltage standard is progressing, the use-fulness of a standard cell and the heavy demand for itscharacteristics will remain unchanged for the present. Thesaturated type standard cells of acid concentration 0.03-0.05 Ncontaining 10-percent Cd amalgam are the most suitable asstandards to maintain and transfer the unit of voltage. How-ever, they have a fairly large temperature coefficient -40 ,uV/°Cat 200 C, and its improvement is desired. To reduce it, variousmethods have been proposed. One solution is the unsaturatedcell which is not so stable as the saturated cell. Another is thecell with a negative electrode of composite amalgam studiedby Vosburgh [1]-[3] and Roberts and Hamer [4]. Thepresent study is basically an exploration of this idea. Sincecomparatively good results have been obtained by adding Pbin the preliminary tests on addition elements, the addition ofPb to Cd amalgam has been mainly studied.

319

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, NOVEMBER 1972

CRITERIA OF HIGH-QUALITY STANDARD CELLS

High-quality standard cells such as those used for the main-tenance of the unit of voltage should satisfy the followingrequirements: 1) good reproducibility, 2) long-term stability,3) speedy recovery from the effect of charge and discharge,4) small hysteresis for the variation of temperature. Suchcells can be obtained only by selection from among a numberof cells. Therefore, a selection method that assures theircharacteristics and stability over a long period of time bytests done in a short time is necessary. The various propertiesof cells, namely, electromotive force, stability with time,temperature coefficient, thermal hysteresis, dc and ac internalresistance and its ratio, effect of charge and discharge, effectof vibration, etc., have been analyzed. The rational criteriafor Weston normal cells, as shown in Table I, and convenienttest procedures have been established [5],[6]. In these, thespecial considerations simplifying the tests are taken, such as

tests of temperature coefficients and hysteresis done at thesame time. The program of temperature in thermal tests isshown in Fig. 1. The tests were done within 15 months aftermaking the cells. The cells satisfying the preceding criteriaare conserved as spares and then after finally considering theirstability, the selection is made according to the purpose of use.

Actually the cells with stability less than 0.3 ,uV/year or so are

selected for precision use. These rules are used to evaluate thequality of the lot as well as to select the cells. Therefore, theyhave been applied for the present study except for the criteriaof EMF and temperature coefficient.

1) Value of EMFat 200Ca

2) Stability of EMFwith time

3) Temperature coefficient

4) Thermal test

5) Discharge test

6) Internal resistance

7) Other test

neutral 1.018 650 V ± 30 uVa

0.03 N 1.018 630 V ± 30 AVacid lo osN 1.018 620 V± 30 AVannual change: ± 1 ,uV yearaseasonal hysteresis (difference betweenmaximum and minimum of observedvalues over a year): ±2 ,V

fluctuation at time of measurement:< l,uV

value of coefficient a2O: -37.9 - -43.8MV/oCa

deviation from formula of second order(obtained by least squares method),namely, difference between measuredand calculated values: ± 10 MV

hysteresis in temperature rise or fall:E1(20) -E(20) ± 5 ,uVE2(25) - E(25) ± 1O0 AV

recovery of EMF after thermal test:E3 (20) E(20) ± 5 MV in a weeka

recovery of EMF after discharge:+5 MV at 2 min after 3 min ofdischarge (0.1 MA)a

dc internal resistance Rdc: < 1000 Qaratio Rdc/Rac: < 1,30asensitivity at time of measurement:better than 0.5 MV

change of EMF due to simple movementor slight vibrations in short time:+ 5 MV

appearance (penetration of oil, changeof color, gas, air bubble, organic sub-stance, lead wire, etc.): in normalstate

aMost important factor.

PREPARATION OF SAMPLES

The conditions of preparation of samples, namely, glass con-

tainer, materials used and their purification, assembling, etc.,were the same as those of normal Weston cells of saturatedtype [7] except for the composition of amalgam used in thenegative electrode. All of the saturated solution of CdS04 con-

tained 0.05 N H2 S04. The metals used, Cd, Pb, and Bi, are ofthe highest grade obtained commercially (guaranteed reagent)and their purities are as follows: Cd (99.9999 percent),Pb (99.99 percent, so-called silver free) and Bi (99.99 percent).The Cd-Pb or Cd-Pb-Bi composite amalgam was prepared bymixing the metals and by heating. A total of 133 cells withdifferent compositions of amalgams have been prepared, con-

taining 28 normal cells for comparison, divided into 4 groups

frQm June 1969 to April 1972. A group with the same com-

position normally contains 5 cells. The typical characteristicsof samples are shown in Table II and the EMF versus tempera-ture curves in Fig. 2.

EFFECT OF THE ADDITION OF Pb TO Cd AMALGAM

The experiments have been made by changing the compositionof Cd-Pb amalgam in the ranges of 0.5-12.0 weight percent(wt %) Pb, 6.0-12.0 wt % Cd, and the balance Hg. The relationsbetween contents of Cd or Pb and the temperature coefficientat 200C are shown in Fig. 3, and the comparisons of reproduci-bility and stability in Figs. 4 and 5. In the compositions of

E(35)35 F-

30

(oc)

25E(25) E 2(25)

E3 (20)20 _

1 5

1 2 3 4 5 6 7 8 15(day)

Fig. 1. Program of temperature in thermal test.

amalgam containing more than 8 wt % for Cd and 2 wt % forPb, an almost constant temperature coefficient (about- 17 MV/0C at 20°C) and an EMF higher by about 1000 ,V thanthat of normal cells, have been obtained. The cells in this range

are reproducible and satisfy the preceding criteria for the mostpart. This lower limit of Pb is closely related to the solubilitylimit of Pb in Hg. Also, as shown in Table III and Fig. 5, noremarkable differences in characteristics and stability havebeen recognized except EMF and temperature coefficient be-tween normal cells and cells containing Pb. Although the cells

TABLE ICRITERIA OF STANDARD CELL OF HIGH QUALITY

If I 4i _

320

HIRAYAMA AND SHIMAZAKI: CELLS WITH Cd-Pb AMALGAM ELECTRODE

TABLE LICHARACTERISTICS OF SAMPLES WITH VARIOUS COMPOSITIONS OF AMALGAM

E m f ToiiA CturL 1hcorrnc I n1ciric ( Rec overy C ornroCelltN3 I,o1t-2ov.')J;Et,. coIffcicnts _ hystoresis(O\') rce i-t nctcsc, of e rm- f OarrnCl

( 2 0 °C ) r ;no(}i\g,,( 21, (!f8I,(7) [(:12,.JF.j dc(0 KLrOC(!l){~RC af tcrrdiSC-Ch d\L1J k. ) '2' 2 2 __ ______(,W)~Pd c(.n,C

Ju ly63 Mcr721,016

A1952 633.02 031.18 - 38.57 -1,079 - 1,31 + 1,12 5390 502.81 1,072 -0.55 10A1954 631,906 30,26 - 3952 - 0.7'986 - 1,17 - 0.17 55?5 5 1 7,o09 1,C69 - 0.65 10

1.019P- 2 646.93 64638- 1733 -0.603 - 2.99 - 1.02 623,5 -S54+O 1,0S5 0 10

July701.018

A2007 623.5s 621,74 -4011 - O§98 - 1.28 - 0,10 607,5 567,1S 1.070 - 055 10

A2009 624.28 621.99 - Z030 -0i3 5 - 1.60 - Q43 5S9.0 559.77! 1.070 - 0.60 101,019

P1- 3 3'7461 374,39 - 4803-0.920 -30,90 -27,52 63".5 591.52 1.073 - 0.50 10

P3-2 640,66 637,31 - 17,08 -Q746 - 0.55 - 0Q65 59270 553.531 1.070 - 0.05 10P5-3 640,51 63791 - 1685 -1.052 - 1.75 - 0.27 6 205 58233 1,065 - 0.40 1 0P7- 4 636.74 637,32 - 17,50 -0,697 - 1,42 - 0,65 584',0 54f3,43 1 075 - 0,60 10P9-4 638.74 6362 - 1756 -0.715 - 0,72 - 0.17 90.0 S46,20 1.062 - 0,50 10

Sept7lP051 087.25 09730 -44924 -0,529 -0022 -18,50 -1Q03 562.0 490Q80 1,145 - 1.40 1 0P22 65i2.72 651.918.40'/ i034 -0,158 +32 , 22 500,0 44220 1131 - 0,90 10P122 661.40 65234 -17785-0634 +Q0012 - 383 -0.36 653,0 589721 1l1 6 - 065 10P361 651,2G 64378-310031-3Q2619 G97 - 2.9 8 + 1,3 3 615;.0 53r 61 1.141 - 1 3 5 6P382 651 20 64729 -184631- 0260-0026 + 4 71 + 2,0 65,5 555.10 1.1 66 - 1.50 8P31 23 65153 64977-18,839L 0552|0Q003 5.18 | 3.17 6Z3,5 536rQ3 1,1 62 -11. 1 2

Set71i

P511 319.30 2 7 8,11 -14. 33 - 1.05 4 +00 1 5 2.8 9 - 1 7,1 5 81 7 5 6 2 ,2 7 1,301 - 01 5 1 0P531 876.50 73 9,01 -10Q;96 -0,302 -0,063 -42,21 -3 2,06 814.5 624,42 1 304 - 3,10 1 0

1.0 21P551 138822 234,48-5,103 -1.599 -Q020 .-56,49 -56,78 1911,5 613.88 3,114 - 3,50 1 0

May721,021

P575 95554+9.721 -6,042±-018O8 -11 2,84 -712.94 11 31.0 603,4i3 1,874. - 3,1 0 10P5104 896.44134743+1.541 -0.311 - 52, 2 -38.568 916.5 604.32? 1.517 -2785 10P5125 899;1632.971 -0147 -0.08 - 4 5,5 6 - 41.34 11 53 5 60371 1,911 - 2, 5 10

I_ III I I ]

P 5 5 1 ( CdlO% -Pb 5 %+ Bi 5 % )

P 571 ( Cd1O0% Pb 5 %+Bi 7 % )

P5122 ( Cd10 % Pb 5 .tBj12% )

15 20 25 °C 30 35

Fig. 2. EMF-versus-temperature curves of typical samples with various compositions of amalgam.

A

II hl (()

Pb B I

0 1-m a1kin-

June69

Juno 70

19

Juno 71

"I

/I

April 72f

5

3579

0.52

1 2333

S5

5

555

3

5

71012

321-

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, NOVEMBER 1972

-10

- 20

- 30

-40

- 50

(V)1 01966

1.01965 _

1.01964

1.0196310 MV

A-v.I* vL 1Wt

1.01937 _

1.01936( b )

Pb 3%1.01910 _

1.01909_

1.0186 3

6 8 10 1Z 14 Cd %

Fig. 3. Relation between temperature coefficient at 200C and contentof (a) Pb or (b) Cd in cells with Cd-Pb amalgam electrode. Fig. 4.

PC Pb)1.01862 ;_.__

%-- -e %-j k -%- Lvj 1-f 4-lP-0.5 P-2 P-5 P-9 Cd

P-1 P-3 P-7 P-12 %Reproducibility of cells with Cd-Pb amalgam electrode.

65

64

(V) 631, 019 6 2

64

63

Cd-Pb Cell

1 0 tl

Test Test ---- 5-a- 5

---'-- 7

A1952

A1954 A200962 - Cd Cellt -A2007

1.01861 1.1 I, , IJuly'69 Jan70 July70 Jan71 July'71 Jan'72

Fig. 5. Comparison of stabilities with time of normal Weston cells andcells with Cd-Pb amalgam electrode. Test shows period in which testof characteristics has been done.

TABLE IIICOMPARISON OF CHARACTERISTICS AND STABILITY WITH

TIME OF NORMAL WESTON CELL AND CELL WITHCd-Pb (5 wt %) AMALGAM ELECTRODE

Normal Weston Cell Cell With Cd-Pb

1.018 V + 1.019 V +EMF July 24, 1969 631.54 ± 1.07 649.23 ± 3.15(+E AV) Sept. 17, 1971 632.36 ± 0.28 648.97 ± 1.20Variation of EMF

for 26 months (AV) 0.82 + 0.93 -0.26 ± 3.30Temperature coefficient(MV!0 C) -39.35 ± 0.60 -17.29 + 0.22

Recovery of EMFafter thermal test (AV) 0.33 ± 1.22 0.59 ± 0.39

Recovery of EMFafter discharge (AV) -0.45 ± 0.14 -0.22 ± 0.20

DC internal resistance(Q2) 548.60 ± 15.0 640.30 ± 35.0

Ratio of internalresistances dc/ac 1.07 ± 0.00 1.08 ± 0.00

Note: Number with sign ± shows rms error of measured values.

TABLE IVSTATISTICS OF THE STABILITY WITH TIME OF CELLS WITH Cd-Pb AMALGAM ELECTRODE

Content of Number of Samples ClassifiedPb Total Number on Basis of Stability with Time (MV/year)

(percent) of Samples ± 0.5 -0.5 - -1.0 -1.0 -2.0 -2.0 -4.0 -4.0 -8.0

3 5 2 1 1 1 05 10 9 0 0 0 17 5 5 0 0 0 09 5 4 0 0 0 1

have been tested by holding at 0°C for a week, abnormalphenomena have not been observed. The statistics of stabilitywith time of cells containing 10 wt % of Cd and 3-9 wt % of Pbare shown in Table IV. They have been calculated on the basisof recent data over one year except for those in the firstperiod of stlbilization and tests of characteristics. The demandof ± I ,V for the stability with time was satisfied at the ratioof 84 percent (100 percent for other characteristics) and this

ratio was not inferior to that of normal cells. Therefore, as aresult of these experiments, it has been concluded that theaddition of Pb is not only harmless but useful, and the mostsuitable composition of amalgam is that of 10 wt % Cd,3-9 wt % Pb, and the balance Hg.The relation between EMF and temperature may be expressed

as follows in the range of temperature 15-350C for cells con-taining amalgam of 10 wt % Cd, 5 wt % Pb, and the balance Hg:

L.

322

.-

IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. IM-21, NO. 4, NOVEMBER 1972

and Pb are constant and that of Bi changed. As shown inFig. 6, the temperature coefficient at 200C changes fromnegative to positive with increasing Bi, and the zero coefficientis obtained at about 6 wt % Bi. However, it has beenobserved that the hysteresis due to thermal cycling becomeslarger with increasing amount of Bi, and the cells containingBi are generally inferior to the normal cells in other character-istics as shown in Table II.

CONCLUSIONAs mentioned above, it has been confirmed that the addition

of Pb has a considerable effect on reducing the temperaturecoefficient, and the cells with Cd-Pb amalgam electrode are

1llS l l l l l appropriate for the purpose of high-precision work, e.g., trans-l 2 3 4 5 6 7 8 9 10 1 1 12 ( Bi %) fer of a voltage standard, in which the cells are enclosed in a

Relation between temperature coefficient at 20°C and content temperature-controlled air bath. The most suitable composi-f Bi in cells with Cd(10)-Pb*(5)-Bi amalgam electrode. tion of amalgam is that of 10 wt % Cd, 3-9 wt % Pb, and the

balance Hg. As for the cell with Cd-Pb-Bi amalgam electrode,F20 - 17.3 X 10-6 (t - 20) - 0.61 X 10-6 (t- 20)2 the zero temperature coefficient at 200C may be obtained, but

r;t an 20eprs,repctvl, h EFsinvlt tits characteristics are inferior to those of normal cells.CtandER Eexpress, respectively, the EMF's in volts at

* 4. REFERENCESOn the other hand, for the normal Weston cell;

Et = E20 - 39.9 X 10-6 (t - 20) - 0.84 X 10-6 (t.-20)

ADDITION OF Pb AND Bi TO Cd AMALGAM

The addition of Pb and Bi to Cd amalgam has been tried tomodify the temperature coefficient in which contents of Cd

[1][2][ 3]

[4]

[5][6][7]

W. C. Vosburgh, J. Amer. Chem. Soc., vol. 47, p. 2531, 1925.-J. Amer. Chem. Soc.,vol. 61,p. 652, 1939.W. C. Vosburgh et al., J. Electrochem. Soc., vol. 111, p. 997,1964.G. N. Roberts and W. J. Hamer, J. Res. Nat. Bur. Stand., vol. 72-A,p. 351, 1968.H. Hirayama et al., 11th C.C.E., CIPM, doc. 10, 1965.H. Hirayamaetal.,Proc. Verb. 12th C.C.E., CIPM, E 69, 1968.Proc. Verb. CIPM, vol. 23B, p. E57, 1952.

A High-Accuracy Digital Instrument Design

for DC MeasurementsLOEBE JULIE

Abstract-A high-accuracy self-calibrating digital instrument designhas been developed in order to make available the inherent efficiencyadvantages of conventional digital meters, but without their limitationsin accuracy, reliability, or certifiability. The new instrument can ex-

tend the advantages of automation to many testing, quality control, andstandards laboratories where heretofore only high-quality manuallybalanced instruments could be used. Automation at this level of accu-

racy can eliminate serious cost bottlenecks in the testing of precisiondevices such as Zeners, resistors, A/D and D/A networks and converters,standard cells, regulated power supplies, voltage and current sources,digital and differential voltmeters and potentiometers, and the dc out-

Manuscript received July 11, 1972; revised July 21, 1972.The author is with the Julie Research Laboratories, Inc., New York,

N.Y. 10023.

put of many precision transducers. The paper describes a number ofapplications of this new type of digital instrument and presents samplesof the type of automatic test data obtained.

THE "digital" (i.e., autobalancing with electrical digitaloutput) voltmeter, ohmmeter, and ratiometer have be-

come key instruments in the field of scientific measurement.The immediate labor-savings advantages, which were obtainedfrom the fast and easy-to-operate digital instruments, led totheir widespread adoption and use in spite of their many per-formance disadvantages. As compared to high-quality manu-

ally balanced potentiometers, bridges, and ratiometers, the

U20.Ct 40

* 30 k

2D0

10 ,

0K

Cd lO%Pb 5 R.

- 1 0 _

- 20

0

Fig. 6.

Et =

where It and 24

323