MEASUREMENT OF POTENTIAL DIFFERENCE 299 



necessary, the cadmium may be purified by distillation at re- 

 duced pressure. 



The effect of variation of the concentration of the amalgam 

 is shown in Fig. 173, which gives the e.m.fs. when various 

 amalgams are tested against 

 one having 14.2 per cent, of 

 cadmium, the electrolyte being 

 saturated cadmium sulphate 

 solution. Variable results were 

 obtained when the percentage 

 of cadmium was over 14.5 per 

 cent. Amalgamated cadmium 

 rods also gave variable results. 

 Identical results (to O'.OOOOl) 

 were given by amalgams con- 

 taining from 6 to 14.3 per cent, 

 of cadmium. 



It may be noted that the 

 stability of the amalgam when FlG - 172. Weston normal standard 

 subjected to variations of tem- 

 perature is influenced by its composition; irregularities in the 

 behavior of the cell were noticed at about 15C. At first it 

 was supposed that these were due to an inversion of the cad- 

 mium sulphate, similar to that of zinc sulphate at 39 but 

 later it was found that they were due 

 to the amalgam, and were obviated by 

 employing a 12.5 per cent, in place of 

 the 14.3 per cent, amalgam at first 

 used. 



The e.m.f. at 20 of the normal cad- 

 mium cell, containing saturated solu- 

 tion, is, when derived from the inter- 

 FIG. 173. Illustrating national ampere and the international 



effect of strength of amal- o h m 1.01830 international volts. 14 Its 

 gam in the cadmium cell. 



e.m.f. at any temperature, t, is 



E t = 20 - 0.0000406[* - 20] - 0.00000095[* - 20] 2 

 + 0.000000001 [t - 20] 3 . 



The advantages of the cadmium cell are low temperature 



