256 Prof. Oliver Lodge on the Seat of the 
for B/C, for C/D. . . and for Z/A, you arrive at the total 
H.M.F. of the circuit A, B, C....A. True, but what then? 
The Volta effect you call A/B is really 3 
air/A + A/B + B/air ; 
that you call B/C is 
air/B + B/C + C/air ; 
and that you call Z/A is 
3 air/Z+ Z/A + A/air. 
Add them up, and you get 
A/B+B/C+. ..4+Z/A, 
which must be the H.M.F. of a circuit by common sense—. e. 
without violent experimental disproof, which no one has ever 
attempted to give. This fact, that the sum of the Volta effects 
equals the sum of the true forces, in a closed circuit of any . 
conducting materials, has nevertheless caused persons to sup- 
pose that air/metal forces are negligibly small. But it is 
clear that they may have any value they like without affect- 
ing the truth of the law. They could only affect it if air/M 
were not equalto —M/air. The experimental proof of the 
summation law, therefore, establishes that air/M is equal to 
—M/air, as well as the important fact that the contact-force 
at each junction is independent of all other junctions of what 
kind soever. 
8. Leaving electrostatic determinations as without bearing 
on the point at issue, let us ask, Is there no direct and straight- 
forward way of measuring the actual H.M.F’. at a particular 
junction without disturbance from other junctions? The 
answer is most clearly given by Clerk Maxwell*, thus :— 
“Sir W. Thomson has shown that if [I is the coefficient of 
Peltier effect or the heat absorbed at the junction by unit 
current in unit time, then JII is the E.M.F. at that junction 
acting with the current. This is of great importance, as it 
is the only method of measuring a local H.M.F. ; the ordinary 
method of connecting up by wires to an electrometer being 
useless. This Peltier measurement is quite independent of 
the effect of contact-forces in other parts of the circuit. But 
the H.M.F. so measured does not account for Volta’s force, 
which is far greater and often opposite. Hence the assump- 
tion that the potential of a metal is to be measured by that of 
the air in contact with it must be erroneous, and the greater 
part of Volta’s H.M.F. must be sought for, not at the junc- 
tion of the two metals, but at one or both of the surfaces 
* ‘Hlectricity and Magnetism,’ vol. i. art. 249. Abbreviated here, 
because so easy of reierence. 
