A SIMPLE DIRECT-READING POTENTIOMETERFOR STANDARD CELL COMPARISONS

BYMARION EPPLEY AND WILLIAM R. GRAY

Of the many types of potentiometer that have been described'there are but two which are available for the direct comparisonof the electromotive forces of standard cells. They are the"slide-wire" type, and the "Feussner" type.

The "slide-wire" type is the better of the two theoretically,on account of the absence of contact resistances in the measuringcircuit. However, it possesses an uncertainty due to the inabilityof adjusting a slide-wire to uniformity of resistance over its entirelength. Also, it is impracticable to manufacture an instrument ofthis design reading to more than five figures. In the range re-quired for standard cell measurements, this makes the fifth deci-mal place an estimated one. This place can be estimated to about20 microvolts upon high-grade instruments. Thus, their precisionis approximately .002%, while their accuracy is usually guaranteedto .02%, at their full scale reading.

The "Feussner" potentiometer is expensive to make owing tothe great number of coils and the need of their being accurately ad-justed to each other to insure a measuring current of constantvalue. The contacts also must be most carefully designed, made,and cared for, if the intended accuracy of the instrument is to bemaintained. White2 states that an error of ten microvolts or moremay result from lack of care of the contacts in the Feussner poten-tiometer as made by Otto Wolff, a maker noted for his elaborateand smooth-working switches. The Feussner potentiometer couldbe made with an infinite number of dials in so far as theoretical

I White, J. Amer. Chem. Soc., 36, p. 1868-1875; Laws, Electrical Measurements,p. 273-288; Watson, Practical Physics, p. 495-498; Griffiths, Methods of MeasuringTemperature, p. 60-66.

2 White, Ref. 1, p. 1871.

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EPPLEY AND GRAY [J.O.S.A. & R.S.I., VI

considerations are concerned. From the practical standpoint,six figures are all that can be secured owing to the total resis-tance being fixed at its upper limit by galvanometer sensitivityand insulation leakage, and at its lower limit by the resistance ofthe dial switches.

There are ways3 in which the residual emf of two opposedstandard cells can be measured to ten microvolts or better with-out any very great accuracy in the resistance and ammeter form-ing the set-up. Difference methods, however, do not give finalresults without calculation, an objection when a large number ofroutine measurements must be made.

If an accuracy of ten microvolts or better is desired, without theneed of calculation, some such arrangement as that described be-low is necessary. In this design the two potential-point featureof the slide-wire type of potentiometer is retained with its ad-vantage of absence of contact resistances in the measuring cir-cuit. The uncertainty due to lack of uniformity in the slide-wireis eliminated by the use of coils throughout. The use of coils ismade possible by limiting the range of the instrument to that re-quired for comparing standard cells. No originality is claimed,except perhaps for the self-checking feature.

DESCRIPTION: The diagram gives all salient features of thepotentiometer circuit. The reproduction of the photograph ofthe top shows the arrangement of dials and switches. The self-checking device is perhaps not so evident, for in adjusting themeasuring current no coils are used other than those of thepotentiometer circuit proper.

Each stud is drilled, on a radius greater than that of its switch-arm, with a hole reamed to receive a plug. A flexible connectorwith plug Si is attached through the standard cell to one pole of adouble-throw switch. Another flexible cord with plug S2is attacheddirectly to the other of this pair of poles. The opposite two polesof the double-throw switch and its two middle poles are connectedas is usual in self-checking potentiometers. When the two plugsare inserted in the holes in the two studs corresponding to the

3 Lindeck and Rothe, Zeitschr. f. Instrumentenkunde, 20, p. 293, 1900.

860

Oct., 1922] POTENTIOMETER FOR CELL COMPARISON

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EPPLEY AND GRAY [J.O.S.A. & R.S.L, VI

electromotive force of the standard, and the double-throw .switchis thrown to the "Standard Cell" side, the cell is in series with the

galvanometer, and is across the resistance proportional to itsvalue. By simply throwing the switch to the E.M.F. position, theunknown is thrown in series with the galvanometer, without dis-turbing the standard cell connections. In this way, convenienceis secured without the need of adjusting a "standard cell coil"to proportionality with the coils of the measuring circuit,

FIG. 2. Top of Potentiometer

The comparison of a cell having a value falling at the upper-most point of the range of the instrument, with a standard whosevalue lies at the lowest limit of the range, presents the worst con-dition for accuracy. An error of 0.1% in the resistance of the1017.0 ohm coil of the potentiometer circuit would, under theabove conditions, introduce an error of 3 microvolts. The samewould be true of a 0.1% error in the same direction in all the re-sistances of the two dials, the 1017.0 ohm coil remaining constant.

862

Oct., 1922] POTENTIOMETER FOR CELL COMPARISON

To secure uniformity of temperature, the instrument is im-mersed to the bottom of the ebonite plate in a stirred oil-bath.

A highly sensitive galvanometer is necessary. Most excellentresults have been secured with a Leeds & Northrup CompanyType HR galvanometer having the following characteristics;sensitivity 2460 megohms; 2.03 mm per microvolt; 102 ohmsresistance, 12 seconds period.

The parasitic electromotive force across the standard cellbinding posts, with the battery disconnected, but the galvanom-eter connected as usual and the key 0 closed, was less than fivemicrovolts, the checking switch being closed at the "StandardCell" position. With the checking switch at the "E. M. F."position, the parasitics across the E. M. F. binding posts wereabout five microvolts. Swinging the long arm over its entirerange of travel twenty times, as rapidly as possible, produced an

E. M. F. of 18 microvolts which in thirty-five seconds sank to 5

microvolts. Swinging the short arm twenty times in the samemanner produced an E. M. F. of 10 microvolts which sank tofive microvolts in thirty seconds. Measurements were made witha Leeds & Northrup Company Type K potentiometer using the"low range."

A storage cell was found to drift too rapidly for satisfactory re-

sults. A standard battery was therefore made differing from thatdescribed by Hulett4 in the following details: A crystallizing dish17 cm in diameter and 9 cm deep was placed in the center of

another crystallizing dish 25 cm in diameter and 12 cm deep. The

inner dish was partially filled with mercury and mercurous sul-fate. Into the annular space molten 1212% cadmium amalgamwas poured to a depth of about 2 centimeters. Crystals of cad-

mium sulfate were placed in both dishes and both dishes filledwith a saturated, acidified solution of cadmium sulfate. A coverwas provided, and suitable connectors of platinum wire in glasstubing.

I Hulett-Phys. Rev., 27, 33.

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864 EPPLEY AND GRAY [J.O.S A. & R.S.I., VI

Two of these cells have proved sufficient.5 After the lapse oftwo hours or more after adjustment of the measuring current,not more than a millimeter deflection of the galvanometer is cus-tomary if the current had been flowing for half an hour before theadjustment. When not in use, the circuit is kept open. Twosuch cells have been in use for 7 months and are still giving thesame service as originally. They have received no attention.

Check-measurements on standard cells, made with this poten-tiometer, by the opposition method of Lindeck and Rothe,6 and bythe Bureau of Standards, usually agree to ten microvolts or better.

Tm EPPLEY LABORATORY,NEWPORT, R. I.

5 Note: These two cells would not operate a Leeds & Northrup Company TypeK potentiometer satisfactorily.

6 See reference 3.