Electromotive Forces in the Voltate Cell. 255 
others, and still more thoroughly and exhaustively by Fara- 
day. It is the easiest possible thing to make a number of 
batteries which shall give a. current without any metallic 
junction whatever. Faraday gives some thirty of them*. 
One more certain proposition we can lay down—viz. that 
whenever a current is produced, the energy of the current 
must be maintained by absorption of heat, or by chemical 
action, or by gravity, or by some other such agent, not by 
mere contact. 
So much being agreed to, what remains as subject-matter 
for controversy? This: A voltaic circuit contains at least 
_ three junctions ; what is the value of the contact force at each 
of them ? and especially to which junction is the major part of 
the observed H.M.F. due? Is it the zinc-acid ? or is it the 
copper-acid ? or is it the zinc-copper? ‘There is no other 
question. The old chemical and contact controversy has died 
out, but another controversy remains. Most physicists pro- 
bably would say today that the major part of the H.M.I’. of 
the cell resides at the zinc-copper junction. This was Volta’s 
view, and this is the view of the text-book writers taught 
by Sir William Thomson. Some few would say at the 
zinc-acid junction, and among them | must confess myself. 
It is no question between contact and something else ; it 
is a question between a feeble energy-less metal-metal con- 
tact, and an active energetic metal-fluid contact with poten- 
tialities of chemical action straining across the junction. 
What is there to distinguish between the two? Hlectrostatic 
experiments with air-condensers prove nothing. They add 
up three H.M.F\s, air/M+ M/M’+M’/air, and give you the 
sum. ‘The experimenters usually assume that M/M’ is what 
they are measuring, but there is no proof to be given in sup- 
port of the assumption, except that if you substitute water for 
air the effect remains almost unaltered ; but then water con- 
tains oxygen as the active element the same as air does. 
Well, then, it may be urged, the effect is the same in vacuo 
and in hydrogen as in air ; and to this I answer, Not proven. 
Can any further assertions be made with reference to elec- 
troscopic experiments as bearing on voltaic theory? Yes, it 
can be asserted that by adding up the Volta effects for A/B, 
* Exp. Res. ii. 2020. Dr. J. A. Fleming describes another of taese 
batteries in Phil. Mag. June 1874, and gives some very cogent and read- 
able arguments in favour of the ‘‘ chemical theory ” of battery E.M.F., sug- 
gesting that the difference of potential between the terminals of a battery 
on open circuit is due to potential chemical combination of the metals 
_ and electrolytes. He does not, however, explain the old Volta experi- 
ment; and, as Prof. Chrystal has pointed out (Encye. Brit., “ Electricity,” 
p. 99), upholders of the chemical theory are bound to explain this. 
