Electromotive Force of different Forms of the Clark Cell. 129 



For this test four crystal cells of different dates were selected, 

 differing as widely as possible (one-tenth of a millivolt either way) 

 from the mean at 15° C. Six other cells were set apart as standards 

 of comparison, and were kept at a constant temperature of 15° C. 

 night and day throughout the series of observations. The cells were 

 all of the same form and dimensions as the B.O.T. cells above 

 described, and were all sealed with marine glue. 



At starting the mean of the test cells at 15° 0. agreed to one 

 hundredth of a millivolt with the mean of the standard cells. The 

 four test cells were then immersed in melting snow, and comparisons 

 of their E.M.F. with the standards were made at intervals of six, 

 eighteen, and twenty-four hours after immersion. For the next twenty- 

 four hours they were kept at a temperature of 5° C, next day at 

 10° C, then for three days at 15° C, then for a day each at 20°, 25°, 

 and 30° C. In this manner the comparisons at each point were made 

 as nearly as possible under similar conditions. 



The small differences which the cells possessed at starting were 

 maintained, within two or three hundredths of a millivolt, throughout 

 the series of observations. They were possibly due to inequalities of 

 age, or to slight differences in the preparation of the solutions and 

 crystals, or to the fact that the platinum wires in the mercury cups 

 were not amalgamated. No systematic difference in their temperature- 

 coefficients could be detected. 



At each point the mean of the observations taken at the end of 

 the first six hours showed a slight lag, as compared with the obser- 

 vations taken at eighteen and twenty-four hours, amounting on the 

 average to nearly two hundredths of a millivolt. It is necessary to 

 remark, however, that a difference of one hundredth of a degree of 

 temperature means rather more than one hundredth of a millivolt ; 

 and that this apparent lag, corresponding to less than 002° C, may 

 have been due to some constant error of observation of temperature 

 under different conditions in the morning and evening. In any case 

 it is evident that the diffusion-lag, if any, is so small that it would 

 be quite useless to consider it in any case, unless the greatest pre- 

 cautions were taken to secure a constant and uniform temperature, 

 and to measure the temperature to at least 0*01 C. 



After the three days at 0°, 5°, and 10° C, the mean of the four 

 cells returned in less than twenty-four hours to within one hundredth 

 of a millivolt of the mean of the standard cells. After three days 

 at 15° C. the mean values were identical. At the conclusion of the 

 observations at 30° C, the cells were replaced in the bath at 15° C. 

 In fifteen hours the mean value had returned to within two hundredths 

 of a millivolt of the standards. 



Some two months after the above series of experiments, a second 

 set of observations at 30° C. was taken with three of the same cells, by 



