118 Prof. H. L. Callendar and Mr. H. T. Barnes. 



Several comparisons of these portable cells were made early in 

 1894 at different dates and under ordinary laboratory conditions* 

 The results, when corrected for temperature by means of the enclosed 

 thermometers, showed irregular differences, often amounting to 

 nearly 3 millivolts. Of a pair of cells enclosed in one brass case 

 together with a thermometer, one would be sometimes higher and 

 sometimes lower than the other, although the utmost care was 

 observed in using them, and they were both necessarily always 

 under closely similar conditions as regards change of temperature. 



The set of Car-hart cells were much better in this respect. They 

 rarely showed irregular differences of more than half a millivolt; 

 but one of the cells, owing to defective sealing, was generally some 

 2 millivolts lower than the others, and has since that time fallen 

 still further. It appeared probable at first that these discrepancies 

 were partly due to inequalities of temperature between the cells and 

 their attached thermometers, and, if so, that they would be insepar- 

 able from portable cells of such a form under these conditions* 

 About the same time, a number of Clark cells were set up by 

 Professor Callendar and some of the advanced students and demon- 

 strators, in accordance with the form described in the Board of 

 Trade Memorandum, as figured in Grlazebrook and Shaw's ' Practical 

 Physics,' p. 576. Every detail of manipulation and construction wa& 

 carefully followed, except that the cells were set up in test-tubes 

 6 inches long, to permit of their being more deeply immersed in 

 water, and that the glass tubes containing the electrodes were pro- 

 vided with mercury cups at the top to facilitate the making and 

 changing of connections. The cells were kept immersed in water in 

 large glass bottles provided with a stirrer and thermometer. 



The cells set up in this way, although made by different students- 

 with different solutions at different dates, were found to agree more 

 closely among themselves than the portable forms, owing probably to 

 the more constant and certain conditions to which they wero 

 exposed. They still, however, exhibited irregular differences, even 

 when exposed to precisely similar variations of temperature, and it 

 was felt that they could not be used with confidence for any work in 

 which an accuracy of one part in 10,000 was desired. 



Some determinations were also made of the temperature coefficients 

 of these cells when exposed to a variation of temperature at the rate 

 of about 10° in two hours. The results were very fairly consistent 

 for each cell, but gave very different values for the mean coefficient 

 between 10° and 20° C. The lowest value obtained was 0'00045, the- 

 highest 0*00069. The value commonly given for these cells is 

 0'00078. They were all saturated cells containing an abundance of 

 crystals, which remained visible on the top of the paste and through- 

 out the mass at the highest temperature. 



