422 PROFESSOR A. SCHUSTER AND MR. W. GANNON ON A 
6. Third comparison of the Cupboard Clark I. with the Standard, the difference 
being denoted by as. 
7. Reading of the thermometer of the Standard Clark, ¢,*. 
From the above measurements we obtain the electromotive forces of our battery in 
terms of the Standard, thus :— 
Set (1) + Set (2) = 20 Standards + 2 (S, + T,). 
After the completion of each experiment for the equivalent, the comparisons are 
carried out in the reverse order, and if 8, and T, denote the quantities corresponding 
to §,, T, we may take the electromotive force of the battery during the experiment 
to be the arithmetical mean of that determined at the beginning and end. The 
relation, written as above, would be 
Set (1) + Set (2) = 20 Standards + (8, + 8, + T, + T,). 
A small correction is necessary owing to the fact that the temperature of the 
Cupboard Clarks may not have been the same during their comparison with the 
battery as it was while they were being compared with the Standard. We may, 
without sensible error, assume that all Cupboard Clarks varied equally, and that, 
therefore, the alteration due to temperature changes, for each cell measured in 
electromotive force, is expressed by the difference of 3 (a, + a3) and 3 (a, + a). 
Hence we correct for the change of the ten cells by adding to 8, the quantity 
5 (a;—a,). A similar correction applies to the final comparison, so that the complete 
relation of electromotive force is given by 
Set (1) + Set (2) = 20 Standards + (8S, + 8, + T, + T,) + 5 (a, — a, + a, — ag). 
The temperature of the Standard to which the equation applies is the mean 
between 4 (¢,’ + ¢,') and 3(t,°+ ¢,°). The temperatures ¢,, ¢, and ¢,, ¢, were practi- 
cally identical in every case, we may take without error $(¢,;-+1,) to be the 
temperature of the Standard during the comparisons. 
The Silver Voltameter. 
The platinum bowl which served as voltameter had a diameter of 9 centims. and a 
greatest depth of 4 centims. A 20 per cent. solution of the nitrate of silver was used, 
the salt being supplied by Messrs. Jonnson, Marruey, and Co., as thrice recrys- 
tallized. The silver plate serving as anode was supplied by the same firm. It was 
7 centims. in diameter and 2 millims. thick. The dish rested during the experiments 
on tinfoil wrapped round a ving of copper, which rested on a tripod, also covered with 
tinfoil (V, fig. 5). The tripod stood on paraftin blocks. The silver plate, protected 
