368 Prof. W. E. Ayrton and Mr. W. R. Cooper. 



" Variations in the Electromotive Force of Clark Cells with 

 Temperature." By W. E. Ayrton, F.R.S., and W. R. 

 Cooper, M.A., B.Sc. Received October 29,— Read Decem- 

 ber 5, 1895. 



The present position of the Clark cell as a standard of E.M.F. is 

 one of such great importance that it is highly desirable to study its 

 behaviour under all working conditions. From time to time during 

 the last few years the Board of Trade has issued specifications 

 relating to its preparation, each one of which was modified according 

 to the results obtained by the use of the one that preceded it. This 

 led finally to the adoption of the cell as a standard, the specification 

 being included in an Order in Council of last year. More recently, 

 in a slightly different form, it has been legally adopted by the United 

 States of America, and it is probable that other countries will soon 

 follow in the adoption of a standard which may already be regarded 

 as universal. 



In this paper we propose to deal with temperature variations and 

 their effect upon the accuracy of this standard. 



When employing a Clark cell for electrical measurements, it is 

 often difficult, to avoid changes of temperature which are sufficient to 

 cause appreciable error if a corresponding correction is not applied. 

 For example, in comparing such cells between themselves at frequent 

 intervals, the differences are generally observed to change from test 

 to test, and, in order that the figures obtained may be of any value, 

 it is of some importance to know how readily variations of tempera- 

 ture are reproduced as variations of E.M.F. in any particular cell. 

 It is very usual to assume that the temperature coefficient amounts 

 approximately to 0'077 per cent, per degree Centigrade, or that the 

 E.M.F. is diminished by O'OOll volt per degree rise of temperature. 

 There is no doubt that such is often the case, at least for considerable 

 changes of temperature at a slow rate. But we have always felt that 

 a good deal of uncertainty might exist as to the extent of this correc- 

 tion, according as the temperature variation is very slow or compara- 

 tively rapid. Of course, large variations of temperature would 

 always be avoided, as they must necessarily be accompanied by a lag 

 in the corresponding changes of E.M.F. ; but, although ordinary 

 precautions may be taken, small variations generally occur. 



The question then arises — Are we justified in applying in full the 

 correction given above for variations of temperature such as occur in 

 practice, or should its value be materially diminished, except when 

 the cells have been subsequently kept at a steady temperature for a 

 very considerable time. It was with a view of obtaining more definite 



