. 
Electromotive Forces in the Voltaic Cell. 263 
pipe at C, and then show that the vessels are differently 
charged ; which is what Volta did. The model does not 
indeed represent the gradual change of potential induced as 
the distance between the condenser-plates increases; and it is 
scarcely worth while to complicate the matter by making a 
more elaborate model. The thickening of the dielectric layer 
of a condenser, when its plates are separated, corresponds 
exactly to the thickening and strengthening of an elastic 
membrane ; and rise of potential in the one case is accurately 
representable by increase of pressure in the other ; but such 
considerations belong to general electrostatics, and have no 
special bearing on our present subject. 
12. This is perhaps the most convenient place to introduce 
the notes or condensed statements which I drew up and dis- 
tributed at the meeting before the discussion. They were 
intended to be critically exact (allowing of course for mistakes 
and possible slips) so as to bear analysis; and hence it is 
probably worth while to reproduce them here with notes and 
comments. 
I. Orthodox Statements believed by the writer to be true in the 
form here set down. 
A. Volta. 
i. Two metalsin contact ordinarily acquire opposite charges”; 
for instance, clean zinc receives a positive charge by contact 
with copper, of such a magnitude as would be otherwise pro- 
duced under the same circumstances by an E.M.F. of about 
8 volt. 
ii. This apparent contact H.M.F. or “ Volta force ”’ is inde- 
pendent of all other metallic contacts wheresoever arranged; 
hence the metals can be arranged in a numerical series such 
that the “ contact-force ’’ of any two is equal to the difference 
of the numbers attached to them, whether the contact be direct 
or through intermediate metals. But whether this series 
changes when the atmosphere, or medium surrounding the 
metal, changes, is an open question : on the one side are ex- 
periments of De la Rive, Brown, Schulze-Berge; on the 
other side, of Pfaff, Pellat, Thomson, Von Zahn }. It certainly 
changes when the free metallic surfaces are in the slightest 
degree oxidized or otherwise dirty. And in general this 
“Volta force’’ is very dependent on all non-metallic contacts. 
iii. In a closed chain of any substances whatever, the 
* Observe that it is not said that two metals in contact acquire different 
potentials. Such difference of potential I believe to be only apparent. 
Compare figs. 14 and 15. 
+ I put Von Zahn on that side because he himself considers himself 
there, and because the great bulk of his experiments lean decidedly that 
way. 
