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



for rise of zero, were found to be very small and irregular, seldom 

 exceeding 0-01° C. 



The second thermometer was by Hicks, divided to twentieths of a 

 degree. Its corrections were found to be very nearly the same as 

 those of the Kew mercurial standard. 



Over the range 0° to 30° C. the changes of zero of these thermo- 

 meters would never exceed 0"01° C, and were, therefore, disregarded. 

 The correction for the length of stem exposed, never exceeding two 

 or three hundredths of a degree, could be applied with sufficient 

 accuracy when required. 



The comparisons were made with a platinum thermometer con- 

 structed of special wire, which has been repeatedly tested by Pro- 

 fessor Callendar, and also by Mr. Griffiths, and by Messrs. Heycock 

 and Neville. The wire is the same as that used in the thermometers 

 made for the Kew Observatory, and its " delta-coefficient " has been 

 taken as 1*50. 



The resistance box used was of special design, reading to 0"0001° C. 

 It was exhibited by Professor Callendar at the May Conversazione 

 of the Royal Society in 1893. The box used at Kew for platinum 

 thermometry has recently been constructed on the same model, and 

 has been described in ' Nature,' November 14, 1895. The Kew box 

 differs chiefly in the use of plugs for mercury contacts, and in the 

 absence of the temperature compensating coils. 



§ 6. Comparisons of Board of Trade Cells. 



With this apparatus many more accurate and careful comparisons 

 of Beard of Trade cells were made. Several new cells, prepared by 

 H. T. B. and by other students, were compared with those a year old. 

 The newer cells were generally found to have a slightly higher 

 E.M.F. than the old, and in general differences of the same order as 

 before were observed, if the cells were subjected to different treat- 

 ment. It was noticeable, however, that B.O.T. cells, prepared about 

 the same time in a similar manner, if kept exposed to similar stable 

 conditions, would generally attain the same E.M.F. , within one or 

 two tenths of a millivolt, after a day or so in the constant tempera- 

 ture bath at 15° C. The importance of keeping cells of this type at a 

 constant temperature has been shown by Griffiths,* who has obtained 

 very consistent results with B.O.T. cells treated in this manner. 



Kahle, on the other hand,f finds differences, amounting to 4 or 

 5 millivolts in some cases, between the nine B.O.T. cells which he 

 tested under constant and similar temperature conditions. Such differ- 

 ences are quite beyond the range of our experience, and we do not 



* 'Phil. Trans.,' A, vol. 184 (1893), p. 388. 

 f ; Wied. Ann.,' vol. 51 (1894), p. 194. 



