Electromotive Forces in the Voltaic Cell. 269 
The air surrounding the metals in contact is in a state of 
slight dielectric strain, such as would be produced by two 
pieces of any one metal of similar size and position, charged 
so as to differ in potential from each other by a volt. 
Zinc and copper plates in contact may therefore be regarded 
as the plates of a condenser, but they form a peculiar con- 
denser, for they are not really at different potentials ; the 
whole step of potential which throws the air into its state of 
dielectric strain is located on their bounding surfaces. 
Let s and s' be the electrostatic capacity of the zinc and the 
copper respectively ; think of them as two independent spheres 
united by a fine wire; and let x be their common potential ; 
then the zinc has, by the fact of contact, gone up 1°8+.«, and 
the copper has gone down #—°8; and, since the quantity of 
electricity which left the one went to the other, it follows 
that— 
s(1'8—2z)=s! (x—°8) 
os os 
pr ere 
There is no necessary relation between s and s’ in general, 
but in the ordinary form of the Volta experiment the two 
plates are of equal size and shape; in which case s=s', and 
x=1°3 volt below the potential of the unconstrained air. 
All this is wholly unlike a condenser investigation. To 
treat it as a condenser we must consider the air-surfaces close 
to the two metals as the plates of the condenser, and we can 
then speak of its electrostatic capacity S in the ordinary way, 
and say that it is in the present case charged with the H.M.F. 
of a volt. 
The quantity of electricity on either plate of such a con- 
denser is § (1°8—°8) or §; hence 
s (1°8—wx)=s' (v—'8)=S (1°8—'8). 
And the general relation between the three 8’s is like that 
of two Leyden jars in cascade, viz., 
or 
me Re 
~ sts! 
If the two pieces of metal are circular disks each of radius 7, 
2 
and at a distance z apart, the value of S is of course - 5 
In Volta’s form of the experiment, two plates are put near 
each other, connected for an instant, then removed from each 
other, and the potential of one of them observed. This 
Phil. Mag. 8. 5. Vol. 19. No. 119. April 1885. U 
