422 



Profs. W. E. Ayrton and John Perry. [Mar. 13, 



conclnsions ; 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 difference of potentials of 

 liquids in contact, still since he employed moist blotting paper surfaces 

 instead of the surfaces of the liquids themselves, the authors considered 

 for that reason alone, if for no other, that his results did not carry the 

 conviction the distinguished position of the experimenter might have 

 led them to anticipate. 



They therefore designed a method and an apparatus for carrying it 

 out, by means of which they could measure the difference of potentials 

 in volts at each separate contact of dissimilar substances in the 

 ordinary galvanic cells, from which they could ascertain whether the 

 algebraical sum of all the contact differences of potential was, or was 

 not, equal to the electromotive force of the particular cell in question. 

 From the results they obtained, and which are given in Papers No-s. I 

 and II, "Proc. Hoy. Soc," No. 186, 1878, they concluded within the 

 limits of their experiments that if AB, BC, CD, &c, were the con- 

 tact differences of potential measured separately of the substances A 

 in contact with B, B in contact with 0, &c, then, any one or more of 



the substances being solid or liquid, if any number A, B, C K 



were joined together, and the electromotive force of the combination 

 AK, measured, the following equation was found true : — 



AK=AB + BC + CD+ +JK, 



which proved that each surface of separation produced its effect inde- 

 pendently of any other. 



Their method by which any single contact difference of poten- 

 tials was measured was as follows : — Let 3 and 4 be two insulated 

 gilt brass plates connected with the electrodes of a delicate quad- 

 rant 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. JSTow 1 and 

 2 are made to change places with one another, 1 being now under 

 4 and 2 under 3, then the deflection of the electrometer needle 

 will give a measure of the difference of potentials between 1 and 2 ; 

 and in the present paper it is proved that in order that the observed 

 difference of potentials in the electrometer quadrants shall be pro- 

 portional to the contact difference of potentials desired to be measured, 

 either there must be perfect symmetry in the induction apparatus 

 before and after reversal of 1 and 2, a condition very difficult to be 

 obtained, or else the plates 3 and 4 must, in addition to being connected 

 together, be also put to earth, or reduced to zero potential, before 

 each reversal, and also the mean potential of the substance under test 



