272 Mr. F. E. Smith on Cadmium Amalgams 



These values have been plotted and give the curves shown 

 in fig. 4 (PI. III.). 



Effect of Temperature Changes on the 

 Weston Normal Cell. 



It must not be concluded from the curves shown in fig. 3 

 that the change in the E.M.F. of a two-phase amalgam 

 towards a cadmium sulphate solution has a very small 

 temperature coefficient. The curves indicate the change in 

 E.M.F. with change of temperature of a Weston normal 

 cell ; that is. the difference between the change of E.M.F. of 

 mercury towards a cadmium sulphate solution (plus mercurous 

 sulphate), and the change of E.M.F. of a cadmium amalgam 

 towards the solution. 



Because of its small temperature coefficient the Weston cell 

 is sometimes used in an unprotected state with respect to 

 temperature changes, and instances have been brought to 

 notice in which one limb of a cell has been 3° higher in 

 temperature than the other limb. This produced a change in 

 E.M.F. of nearly 1 part in 1000. llapid changes in the 

 temperature of a cell are of little importance if both limbs are 

 at the same temperature. 



To determine the temperature coefficient of each limb we 

 have used H form vessels in which the two limbs are 8 inches 

 apart. The tube connecting the limbs is bent in the form of 

 U near to one end ; this construction prevents diffusion of 

 the hot liquid from one limb to the cold liquid of the other. 

 Four such cells were made and their electrodes consisted of 



(1) Cadmium 10 %• . Cadmium 10 %. 



(2) „ „ . Hg 2 S0 4 and Mercury. 



(3) ,, ?) • ,•> }-> 



(4) Mercury and Hg 2 S0 4 . 



At the beginning the cells were placed in ice and their 

 E.M.F.'s determined. The one limb was then placed in a 

 bath which could be raised in temperature, while the other 

 limb was kept at 0°. The results are contained in the 

 following table and shoAv the importance of keeping a cell 

 screened, in order that the difference of temperature of the 

 two limbs shall not be appreciable. 



The difference between the two E.M.F.'s at the same 

 temperature is almost identic .1 with the difference calculated 

 by the use of the formula given by Wolff u . 



