30 
PROFESSORS W. E. AYRTON AND J. PERRY 
Now if it be remembered that the various experiments with the different metals, 
although made at an average temperature of 18° C., were, as mentioned in the various 
tables, made at somewhat different temperatures the one from the other, it will be 
seen that the previous six sums cannot be absolutely nought. We may therefore 
regard them as sufficiently small to prove a considerable amount of accuracy in the 
experiments. 
The numbers given in the two large tables for the contact differences of potential 
of solids with solids, solids with liquids, and liquids with liquids might be employed 
to build up the electro-motive forces of well known one and two fluid galvanic cells. 
As however the summation law referred to at the commencement of this paper may 
be considered as having been established in our two previous papers on the contact 
theory of voltaic action, it is not necessary in the present instance to take further 
examples of it. 
In all our experiments two air contacts enter into our measurements, and the 
summation law gives no indication of the value of the contact difference of potentials 
at these air contacts. For example, suppose it be desired to measure the difference 
of potentials between two substances A and B in contact, either or both of which may 
be solid or liquid, then in reality what we measure is 
(Air, A) + AB + (B, Air). 
If now we measure by the inductive method the difference of potentials between 
B and C in contact, then what we really obtain is 
(AffTB)+BC+(^Air), 
and the sum of these two we have proved in all cases to be equal to what is obtained 
if A and C be joined by B and the difference of potentials between A and C measured. 
What really we have proved is therefore that 
(Air, A)+AB+(B, Air) + (Air, B)d-B(J + (C, Ah) = (Air, A)-(-AB + BC-F(C, Ah). 
And this would be the case whatever the values of the difference of potentials at the 
air contacts, provided they remained constant, so that 
(B7Ah) = ~(Ah7B). 
One way, and the way we hope shortly to employ, to enable us to determine the 
electro-motive force of contact, in volts, of a substance and a gas is by repeating all 
these exact contact experiments in different gases, for if we measure by our inductive 
method the contact difference of potentials of A and B in a gas G then we shall obtain 
GA+AB+BG, 
