Electromotive Force of different Forms of the Clark Cell. 133 



agrees with our value, except that the rate of change of the coeffi- 

 cient is twice as great. For the other cell he found a coefficient 

 12 per cent, smaller with a similar rate of change. His cells, in 

 which the zinc was pushed down into the paste, would be certainly 

 less liable to diffusion-lag than the ordinary B.O.T. pattern, and 

 might possibly possess a different coefficient ; but we think that the 

 difference is most probably to be explained by diffusion-lag, and in 

 that case the higher value would be the more correct. 



Grlazebrook and Skinner find the value 0*00076 for the mean 

 coefficient between 0° and 15° C, under conditions in which diffusion- 

 lag would be approximately eliminated. Our value between these 

 limits is 0-000773. 



Our value for the coefficient at 15° C, namely, 0' 00083 7, is also in 

 fairly close agreement with the value 0*00082 for an H-cell at 15° C, 

 found by Fleming. The lower values obtained by many observers 

 for ordinary B.O.T. cells, are doubtless vitiated by the effects of 

 diffusion-lag, and are, in this respect, in agreement with our own 

 results for such cells under similar conditions as given in a previous 

 section. 



The observations of Kahle* are probably the most systematic. 

 He finds for the E.M.F. in volts at t° C. the formula 



E, = E 15 -0-00116(£-15)-0-00001(£-15) 2 (K). 



The difference between this formula of Kahle and the linear 

 formula (L) is shown by the dotted curve in fig. 2. It will be seen 

 that the agreement with our observations is very close between 25° 

 and 30° C, but that the value of the coefficient at 15° 0. is some- 

 what smaller. Below 10° C. the divergence is very marked. The 

 formula of Kahle gives a change of only 15"15 millivolts between 0° 

 a,nd 15° 0-, corresponding to a mean coefficient of 0"000704, values 

 which are evidently much smaller than those given above. 



We do not think, however, that it is necessary to assume that 

 there is any real difference of behaviour between our cells and those 

 tested by Kahle. The discrepancy is more probably to be explained 

 by the fact that the observations on which the formula of Kahle is 

 founded were taken between the limits 12° and 28° C, under condi- 

 tions less favourable to the cells. Between these limits, so far as we 

 are able to judge, we are in agreement with Kahle, within the limits 

 of accuracy of his observations. Kahle does not give any detailed 

 observations or any statement of the probable error of his results, but 

 it is possible to form a general idea of the limits of accuracy from the 

 account which he gives of his method. The cells were kept immersed 

 in paraffin baths, regulated by means of " Rohrbeck " thermostats. 

 The temperature seldom varied more than one degree from day to 

 * ' Wied. Ann.,' vol. 51 (1894), p. 197. 



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