128 Prof. H. L. Callendar and Mr. H. T, Barnes. 



with the mercurous sulphate, the filtrate will remain clear when 

 cooled to 0° 0. It appears probable that if a Clark cell is heated 

 above 30° C, or is made from paste which has been so heated, its 

 E.M.F. may be affected by a similar cause. The temperature of 

 30° C. appears, however, to be a perfectly safe limit. It is not 

 unlikely that the decomposition is determined by the presence of 

 zinc oxide, as we have rarely observed changes of more than one or 

 two tenths of a millivolt even after heating a cell to 50° C. 



§ 12. Tests of B.O.T. Crystal Cells. 



Several of these crystal cells were made by H. T. B., and later by 

 the advanced electrical students in the ordinary course of their 

 work. The cells so set up at different times by different students 

 were rarely found to differ under any conditions by so much as the 

 tenth of a millivolt from the mean at any given temperature. If, as 

 occasionally happened, a new cell, within an hour or so of sealing up, 

 was found to have an E.M.F. as much as two tenths of a millivolt 

 too bigh, it was short-circuited for half an hour or so with a piece 

 of copper wire. This procedure invariably had the effect of reducing 

 the E.M.F. to its normal value. 



It might naturally be supposed that with so small a quantity of 

 solution, these cells would be seriously affected by short-circuiting. 

 We have found, on the contrary, that they are much less affected 

 than the ordinary B.O.T. form or than unsaturated cells. The 

 crystal cells on short circuit were found to give a current of about 

 5 or 6 milliamperes, falling gradually to 2 or 1 in the course of an 

 hour. On removing the short circuit the cells instantly recovered to 

 within a millivolt of their normal value, so quickly, in fact, that it 

 was found impossible by the balance method to obtain any inter- 

 mediate readings showing the rate of recovery. In less than five 

 minutes the value had generally recovered to within a tenth of a 

 millivolt of the normal. 



The ordinary B.O.T. cells of the same size, containing clear solu- 

 tion, were found to give a similar current on short circuit, but the 

 recovery was never so rapid or perfect. 



§ 13. Temperature Coefficient of Crystal Cells. 



Having satisfied ourselves by various preliminary trials that the 

 crystal cells were practically free from diffusion-lag, and finding that 

 the temperature coefficient between 15° and 0° 0. appeared to be 

 somewhat higher than that given by Kahle and other authorities, we 

 determined to make a systematic series of observations under definite 

 and uniform conditions. 



