Ss 
Electromotive Forces in the Voltaic Cell. 163 
Fig. 5.—End view of Ayrton and Perry’s Apparatus. 
On the appearance of Clifton’s paper the year after, they 
issued a strongly worded claim* in respect both of priority 
and completeness—a claim which seems to me well established, 
for their results are the most comprehensive yet obtained, 
and the energy needed to devise, construct, and use such an 
apparatus as the one they depict must have been immense. 
A convenient summary of their numbers is to be found in 
Hverett’s ‘ Units,’ second edition. The main result achieved 
by them is the experimental establishment of the summation 
law for all substances (this is not to be confused with Volta’s 
summation or series law, which is only applicable to metals), 
viz. that the total E.M.F. of a closed circuit of any number 
of substances may he reckoned by adding up the Volta forces 
observed electrostatically for every pair of substances in con- 
tact. This law is, it seems to me, for reasons given later 
(§. 7), very probable theoretically ; but still it was quite 
essential to have it experimentally established, especially as 
Ayrton and Perry point out that it is often called in question 
without good ground. The establishment of this law is, | 
say, perhaps their main work in this matter, besides the 
: is — and Perry: Letter published in 1877 by Meiklejohn (Yoko- 
ama). 
