OX THE CONTACT THEORY OF VOLTAIC ACTION. 1 7 
Since a preliminary notice of our first paper was communicated by Sir William. 
Thomson to the British Association in 1876, Professor Clifton has published one 
“ On the Difference of Potential produced by the Contact of Different Substances; 
but in spite of the exact workmanship of Professor Clifton’s apparatus and the pro¬ 
bable accuracy of the results attainable with it, it nevertheless possesses the same 
objection as the instrument employed by Gerland, namely, that it was not possible to 
measure directly with it the difference of potential of two liquids in contact. The 
results given by Professor Clifton are, as far as they go, generally in accord with 
those contained in our paper. 
II. 
The experiments of which the results form the subject of the present communication 
were made during 1877 and 1878, and the new apparatus employed, although gene¬ 
rally the same in principle as that previously used, possessed many important modifi¬ 
cations suggested by experience. 
The method of measurement was as follows! Let 3 and 4 be two insulated gilt- 
brass plates connected with the electrodes of a delicate quadrant electrometer. Let 
1 under 3 and 2 under 4 be the surfaces whose contact difference of potential is to be 
measured. 3 and 4 are first connected together and then insulated, but remain con- 
nected with their respective electrometer quadrants. Now, 1 and 2 are made to 
change places with one another, 1 being now under 4 and 2 under 3, then the de¬ 
flection of the electrometer needle will give a measure of the difference of potentials 
between 1 and 2. 
Fig. 1. 
For before reversal let (fig. 1) the potential of the electrometer needle be N, of the 
induction plates and of the quadrants be B, of the substance 1 be A, and of the sub¬ 
stance 2 be A+a, so that a is what w r e desire to measure ; 
* Proc. Roy. Soc., 1877, vol. xxvi., p. 299. 
f It need Hardly be noticed tHat an inductive and not a conductive method of experimenting must 
necessarily be employed since the latter would introduce always unknown differences of potential. 
MDCCCLXX X. I) 
