258 Prof. Oliver Lodge on the Seat of the 
and mere contact, he refers to Helmholtz* and Clausiusf, 
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 oxidizability 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 H.M.F. of a pile is that 
represented by chemical action, and since by experiment vol- 
taic contact-forces have much the same values as the H.M.F. 
of piles, it follows that there is some vague relation between 
A/B and the heats of combination, 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 different 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 sur- 
face 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 sur- 
face 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 
H.M.F. of an element of a liquid pile formed by these two 
* Die Erhaltung der Kraft, p. 47, where Helmholtz develops Volta’s 
original hypothesis 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. This view he returns to in his Faraday 
Lecture 1881, where also he refers to Berzelius’s electrical theory of che- 
mical 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. 
} Die mechanische Behandlung der Elektricitat, chap. vii. §§ 2 and 38, 
where Prof. Clausius follows up the above idea by considering the réle 
which heat plays in the matter, and thus hypothetically explains the 
Peltier effect also. 
