98 



Comparison of Clark and Weston Cells. By S. N. Taylor. 



A great deal of work of very excellent eliaracter has been done upon the 

 Latimer Clark Standard Cell by Prof. Glazebrook, Prof. Carhart, Prof. Kahle, 

 Lord Rayleigh and others, and by them the merit of the cell has been well estab- 

 lished. 



It has been shown by them that the cell can be made so that, under favorable 

 conditions, it will v;iry in E. M. F. less than one part in a thousand, even when 

 made by different persons and of materials obtained from various sources. It has 

 also been .shown that with proper care the cell maintains its potential indefinitely, 

 and forms a very excellent standard of electro-motive force, which is both moder- 

 ately portable and cheap. 



It is well known, however, that this standard of potential has at least one 

 very serious drawback, namely, it has a very large temperature coefficient, and 

 the E. M. F. of the cell varies considerably for slight changes in temj)erature. 

 Moreover the coefficient may not be the same for different cells, or may be differ- 

 ent at different temperatuies even in the same cell, if the temperatures considered 

 are not near together. Therefore the coefficient for any cell can be exactly de- 

 termined only by experiment on that particular cell, and must be ascertained for 

 all ranges of temperature to which the cell is likely to be exposed. It is also true 

 that changes in temperature in the cell can not be detected easily and accurately,, 

 and hence arises some doubt as to the actual E. M. F. of a Clark cell at any 

 particular instant. 



Methods have been proposed for obviating tiiis difficulty, but for want of 

 space we must omit them here. It goes without saying, however, that if we could 

 find another cell having the same excellencies as the Clark in all respects, and 

 not having this defect in temperature coefficient, it would be a decided advantage. 



The Cadmium cell, recently invented by Mr. Edward Weston, has attracted 

 considerable attention, and so far as our observations go, it possesses these very 

 qualifications. For the past three years we have spent considerable time in test- 

 ing the merits of this cell as compared with the Clark Standard Cell. 



To do this we made a number of Clark cells according to the latest instruc- 

 tions given by the English Board of Trade, as founil in the Philosophical Trans- 

 actions for 1892. We also made a number of Weston cells similar to the Clark, 

 except that in the Cadmium or Weston cells Cadmium and Cadmium-Suh^hate 

 took the place of the Zinc and Zinc-Sulphate of the Clark. 



