1877.] On the Contact of different Substances. 299 



IV. " On the Difference of Potential produced by the Contact of 

 different Substances." By R. B. Cliftox, F.R.S., Professor 

 of Experimental Philosophy in the University of Oxford. 

 Received May 22, 1877. 



In order fully to determine the origin of the difference of electric poten- 

 tial which is exhibited at the terminals of a voltaic element, and on which 

 the electromotive force of the element depends, it is necessary to inves- 

 tigate the difference of potential produced at each contact of the different 

 Substances contained in the element. 



This difference of potential was approximately determined by Volta in 

 certain cases of contact of two metals, and the investigation of the electric 

 condition of metals in contact has since been more fully studied by Kohl- 

 rausch and others. 



The difference of potential arising when a metal and a liquid are 

 brought in contact was pointed out by Volta. and has since been inves- 

 tigated by Becquerel, Peclet, Pfaff, Buff, Kohlrausch, and others ; but the 

 results obtained have in most cases merely shown that a difference of 

 potential exists, and indicated the sign of this difference. 



The methods of observation adopted by Becquerel, Peclet, and Pfaff do 

 not indeed deal with the simple case of the contact of a given metal and 

 a given liquid, but introduce more than one active contact : and the 

 method of Buff, which Kohlrausch has also employed, is objectionable 

 from the fact that, in addition to a metal-liquid contact a metal-glass or 

 metal-lac contact has been introduced ; and this latter contact is, I believe, 

 far from inoperative, producing, indeed, a very considerable difference of 

 potential. 



In endeavouring to study this subject I have suppressed the solid 

 dielectric and used a condenser having its opposed surfaces of metal and 

 liquid separated by a plate of air, and have been careful to introduce only 

 a single operative metal-liquid contact. 



Although circumstances have prevented me from making much pro- 

 gress with quantitative measurements — and. indeed, the experiments have 

 indicated that a change must be introduced into the apparatus emploved 

 before satisfactory measures of the difference of potential in certain im- 

 portant cases can be obtained, this difference being much less than pre- 

 vious investigations had led me to expect — yet I venture to lay before 

 the Society a short account of some of the results obtained, and I hope at 

 some future time to be able to communicate a more complete investigation 

 with reliable quantitative determinations. 



Por the insulation of the apparatus, on which so much depends, I have 

 employed a modification of the form of insulator suggested by Sir AV. 

 Thomson, which seems to me more convenient than the form in common 

 use, while it equals, if it does not surpass, the latter in efficiency. 



Pig. 1 represents one of these insulators, which consists of a glass 



