16 
PROFESSORS W. E. AYRTON AND J. PERRY 
Regarding objection No. 2, great vagueness existed as to whether the contact 
difference of potentials between two substances, A and B, which we shall call for 
brevity AB, was a constant depending only on the substances A and B and the tempe¬ 
rature, or whether, one or both being a liquid, it was a variable dependent upon what 
other substance was in contact with either. Some writers regarded it as a variable A 
GerlandI considered he had proved it constant when one or both of the substances 
was a solid. But, first, the agreement of the value of the electro-motive force of each 
of his cells with the algebraical sum of the separate differences of potential at the 
various surfaces of separation, which was the test of the accuracy of his theory, is so 
striking, and so much greater than polarisation, &c., usually allows one to obtain in 
experiments of such delicacy, that one cannot help feeling doubtful regard in g his 
conclusions ; secondly, his apparatus did not allow of his experimenting with two 
liquids in contact, consequently he could not legitimately draw any conclusion in this 
latter case. And although Kohlrausch had made some few experiments on the dif¬ 
ference of potentials of liquids in contact, still, since he employed moist blotting-paper 
surfaces instead of the surfaces of the liquids themselves, we felt, for this reason if for 
no other, that his results did not carry the conviction the distinguished position of the 
experimenter might have led us to anticipate. 
In fact, the inability of all experimenters to measure directly and accurately with 
their apparatus the contact difference of potentials of a solid and a liquid, and especially 
of two liquids, caused the whole theory of voltaic action to remain in a vague and 
incomplete state. It therefore appeared to us, in 1875, desirable to design an appa¬ 
ratus and carry out a series of experiments on this subject; and the results obtained 
during the following year will be found in our papers Nos. I. and II. (Proc. Roy. Soc., 
No. 186, 1878). Our method consisted in measuring directly the difference of poten¬ 
tials in volts at each separate contact of dissimilar substances in the ordinary galvanic 
cells, and then ascertaining whether the algebraical sum of all the contact differences of 
potential was or was not equal to the electro-motive force of the particular cell in 
question. The result obtained was that, within the limits of experiment, if AB,| BC, 
CD, &c., were separately measured (any one or more of the substances being solid or 
liquid), then if any number A, B, C, D . . . . K were joined together, and the electro¬ 
motive force of the combination AK measured, the following equation was found 
true 
AK=AB+BO+CD+_+JK. 
Which proved that each surface of separation produced its effect independently of any 
other. 
* F. Jenkin, Elec, ancl Mag., p. 44, 1873 
t Gerland, Pogg. Ann., Bd. cxxxvii., s. 552, 1869. 
+ AB, as previously defined, being tlie difference of potentials between A and B in contact witb one 
another, but neither in contact with any other conductor. 
