MR. F. E. SMITH ON THE NORMAL WESTON CAPMir.M CELL. 



4U9 



the mean fall was 100 microvolts, and there was evidence that they were still falling. 

 These three cells contained pastes prepared by Dr. GUTHE. On Nov. 10, 1906, four 

 other cadmium cells were prepared, and contained mercurous sulphate supplied by 

 Dr. HULETT. By Jan. 19, 1907, the mean fall of these cells was 0-00049 volt; on 

 the same date their average E.M.F. was about '00023 volt lower than that of the 

 three cells previously considered. 



Dr. GUTHE also gives the values of some Clark and cadmium cells set up with 

 electrolytic and chemically prepared pastes by Professors CARHART and HULETT. The 

 Clark cells appear to have remained constant since their construction in 1904, but 

 some of the cadmium cells have fallen by 3 to 4 parts in 10,000. In some cases more 

 than half of this change took place during the first year. 



The results obtained at the National Physical Laboratory are, on the whole, 

 decidedly in favour of the constancy of the cell, and tend to show that the fall in 

 E.M.F. of certain N.P.L. cells is due to the mercurous sulphate in them being 

 somewhat abnormal when they were set up. An investigation of the pastes of 

 abnormal cells appears to be desirable, and may possibly lead to some explanation 

 of the want of constancy which the foregoing statement shows has been noted by 

 some observers. 



E.M.F. of freshly prepared Cells. 



When mercurous sulphate is freshly prepared and apparently free from acid, if 

 cells are set up with it as a depolariser on the same day as that of the precipitation 

 they do not usually take up their normal value immediately. They are sometimes 

 high at first, but fall rapidly in E.M.F., sometimes attaining their normal value 

 within a few hundred thousandths of a volt in a few hours, but more often an interval 

 of several days is required. An example of this is afforded in the case of B 151. 

 This was completed at 2.15 P.M. on the same day as the depolariser was manufactured ; 

 it was inserted in a constant temperature bath, and observations of the E.M.F. were 

 immediately made. At the same time another cell, B 149, was completed ; in this the 

 depolariser was mercurous sulphate whicli had been prepared three weeks previously 

 and had since remained in contact with saturated cadmium sulphate solution. This 

 cell attained its normal value almost immediately. 



TABLE V. 



VOL. OCVH. A. 



3 O 



