228 Weston Cell as a Standard of Electromotive Force. 



If, however, the temperature is below the freezing point 

 of A, as well as below that of B, the relative positions of 

 AM and A'M' will be reversed and it will now be impossible 

 to find values of x and ?/ for which /x a , jxd and jjli>, //,&' 

 respectively, are equal. In other words, one-phase solid 

 mixtures only will be stable *. 



If it is assumed that the /ac variations for the mixtures 

 of Hg and Cd used in Weston cells are of the form repre- 

 sented (diagram matically) in fig. 5, it can at once be seen 

 why the amalgams exhibit the properties which have been 

 discussed. 



§ 15. The temperature coefficients of cadmium-mercury 

 cells. — The efficiency of the Weston cell as a standard does 

 not depend only on the fact that the chemical composition of 

 the amalgam can vary within considerable limits without 

 producing moie than a few millionths of a volt difference in 

 the electromotive force ; but also upon the extreme smallness 

 of the effect of temperature change near 0° C. 



It is instructive to consider how the existence of the two- 

 phase amalgams happens to be the cause of the second 

 phenomenon. 



The data of Table XL (I. c. p. 273) show that the smallness 

 of the temperature coefficient is not due to absence of heat 

 exchanges with the surroundings during isothermal working 

 of a two-phase cell, but to the fact that these nearly balance 

 each other. Somewhere near 5° 0. there is an exact balance 

 and the temperature coefficient vanishes. 



Above 5° C, the temperature coefficient has a small 

 negative value. This means that slightly more heat escapes 

 from the cell (during action) than it absorbs from the outside. 

 The presence of the two-phase amalgam reduces the net loss 

 of heat. For, during the working of the cell, the average 

 percentage of Cd in the anode must diminish and the 

 equilibrium compositions of the coexisting phases can only 

 be maintained by reduction in the amount of the richer 

 (solid) phase. This will occasion absorption of heat and so 

 reduce the net amount evolved. 



Below 5° C, the absorption just mentioned slightly 

 overbalances the remaining effects and the temperature 

 coefficient is positive. 



* Of. § 5 above. 



