ELECTROMOTIVE FOKCES IN THE VOLTAIC CELL. 495 



copper, zinc, air, copper, and is the sum of three EM.F.'s, only one of 

 which has yet been measured.' 



With every word of Maxwell I coi'dially agree. 



9. While on the theoretical aspect of the subject it may be well to see 

 what Pellat, as one of the best experimenters on it, has to say. Pellat 

 substantially observes as follows : — 



' Does the apparent difference of potential between two metals in 

 contact indicate a real difference of potential between them ? In all 

 rigour, Xo ; but the slight variation of its value when different gases or 

 even liquids are used renders it extremely probable that there is such a 

 real difference of potential, and that it is very nearly what is measured 

 in electroscopic experiments.' 



As to difficulties connected with energy considerations and mere 

 contact, he refers to Helmholtz, 1 and Clausius, 2 who, he says, relieve him 

 of all responsibility on this head. 



The fact that the voltaic order of the metals is much the same as their 

 order of oxidisability must have struck nearly everybody, and must also 

 have been felt as a difficulty by the upholders of the efficacy of mere 

 contact. Pellat considers he disposes of it thus : — ' Since the E.M.F. of a 

 pile is that represented by chemical action, and since by experiment voltaic 

 contact forces have much the same values as the E.M.F. of piles, it follows 

 that there is some vague relation between A/B and the heats of com- 

 bination, say of substitution of one metal for another in a salt (as in a 

 Daniell).' 



He sums up his experimental conclusions as follows : — 



(1) ' Two diffei'ent metals united metallically are covered, in the state 

 of equilibrium, with electric coats of unequal potential. 



(2) ' This difference of potential only depends on the superficial coat of 

 metal. It changes notably when the surface is mechanically scratched, 

 becoming always more positive. As the scratching effect disappears with 

 time so does the extra difference of potential. The state of polish of the 

 surface is immaterial, but traces of foreign substances, forming a coat so 

 thin as to be invisible, are able to modify the value of the observed effect 

 enormously. 



(3) ' The effect depends somewhat on temperature. 



(4) ' The pressure and nature of the gas surrounding the metals have 

 a very distinct but extremely feeble influence, but, since the effect produced 

 is a lagging one, it is probably due to some secondary cause, and it is 

 probable that the difference of potential is really independent of the gaseous 

 dielectric. 



(5) ' The difference of potential between the electric coats on two 

 metals united metallically has the same value as the E.M.F. of an element 

 of a liquid pile formed by these two metals, provided that the E.M.F. is 



1 Die Erlt.altung der Kraft, p. 47, where Helmholtz develops Volta's original hypo- 

 thesis about an attraction of matter for electricity, of an amount depending on the 

 kind of matter, so that it gets pulled one way or another across a junction of two 

 dissimilar substances. He points out that the Volta effect is explained if zinc be 

 granted a stronger attraction for electricity than copper has. Tins view he returns to 

 in his Faraday Lecture 1881, where also he refers to Berzelius' electrical theory of 

 chemical affinity. The opinions of Professor Helmholtz are too weighty to be merely 

 referred to in a footnote, but we may have occasion to consider them later. 



1 Die incchanisclie Bchandlung der Electricitdt, chap. vii. §§ 2 and 3, where- 

 Prof. Clausius follows up the above idea by considering the role which heat plays in 

 the matter, and thus hypothetically explains the Peltier effect also. 



