MEASUREMENT OF POTENTIAL DIFFERENCE 295 



Board of Trade Cell. Little careful work was done on the 

 Clark cell until 1885, when Lord Rayleigh investigated a form of 

 cell practically similar to that shown in Fig. 168, which is the 

 Board of Trade cell of 1894. Rayleigh found that the e.m.f. of 

 the cell at any temperature could be expressed by the formula, 



E t o = E u *(l - 0.00077[i - 15]); 



and gave 1.434 as the value of the e.m.f. at 15; subsequent 

 investigation has shown that this figure is too high by nearly 

 0.1 per cent. 



-Marine Glue 



Zinc Rod 



Cork 



-Air Bubble 



Zinc Sulphate 



Solution 



Sulphate 



Crystals 

 Paste 



Mercurous Sulphate 

 ^Zinc Sulphate 



Mercury 



FIG. 168. Board of Trade 

 standard Clark cell, 1894. 



Marine Glue 



Zinc Rod 



inc Sulphate 

 Solution 



Saturated at O C. 

 Cork 



ercurous Sulphate 

 Zinc Sulphate 



Mercury 



FIG. 169. Carhart-Clark 

 standard cell. Used as a work- 

 ing standard before the intro- 

 duction of the Weston cell. 



Rayleigh showed that variations in the e.m.f. were due to im- 

 purities in the materials, and, in this form of cell, to the fact that 

 the zinc was so placed that it was not covered by zinc sulphate 

 solution of uniform density; this greatly retarded the response of 

 the e.m.f. to a change of temperature. The first effect of a 

 decrease in temperature would be to cause crystallization; this 

 requires time, consequently the density in the neighborhood of 

 the crystals lags behind the temperature change and a still 

 longer time must elapse before the solution becomes uniform by 

 diffusion. This accounts for the lack of concordance in the early 

 values of the temperature coefficient. This lag in the e.m.f. 

 becomes greater as the cells grow older, especially if they are 



