Variations in the E.M.F. of Clark Cells ivith Temperature. 381 



what in value when correcting for actual variations, unless these 

 take place with extreme slowness. In fact, it would appear that a 

 more suitable value for the temperature coefficient, for use under 

 ordinary working conditions, would result from such curves as those 

 given above, although it would not be the true value. It must,, 

 however, be remembered that cells differ among themselves some- 

 what as to the value of the coefficient. 



The cells were carefully examined, but, although small differences 

 were apparent, their appearance furnished no characteristics by 

 which the observed phenomena might be accounted for. As already 

 mentioned, the majority of the cells were constructed at the same 

 time, and many were precisely alike as far as could be judged, for 

 example, W, X, Z. 



It has for some time past been suspected that the lag of the 

 E.M.F. of a Clark cell behind the temperature was due not merely 

 to the mean temperature of the cell itself lagging behind that of the 

 bath, but to time being necessary for the variation of the amount of 

 crystals in solution and for diffusion to take place. 



How far this cause of lag is diminished by the use of the H type 

 of cell, where there is only one horizontal surface of contact between 

 metal and liquid, we are about to determine experimentally. 



In conclusion, our thanks are due to Mr. W. Gr. Rhodes, as already 

 stated, and also to several students, among whom we would mention 

 Mr. F. S. Grrogan, who gave much time to the work. 



There appears to be a general tendency for the E.M.F. of cells set 

 up in accordance with the final specification issued by the Board of 

 Trade to fall somewhat below the normal value after the lapse of some 

 months. For example, the E.M.F. of cells set up in October, 1894, 

 although, as a rule, not differing from 1*434 volts at 15° C. by more 

 than 1 part in 3000 when tested soon after construction, is now low 

 by fully 1 part in 1000. The same may be said of cells set up in 

 November, 1894. But the E.M.F. of cells constructed in June, 1895 

 although falling, is still not low by more than about 1 part in 2000. 



