260 Prof. Oliver Lodge on the Seat of the 
of contact. There is, then, at the junction not only the con- 
tact-force H, but also the equal opposite force —dV, due to 
the difference of potential established. Hither of these forces 
alone would resist or aid the passage of electricity across the 
junction and so give rise to a Peltier effect, but both together 
will do nothing of the sort; and so if there be any Peltier 
effect, it must be some small residual phenomenon, or it must 
be due to some other and totally distinct cause*. 
Professor Ayrton’s way of putting the argument, which I 
think he said he got from Sir William Thomson, was some- 
thing like this. When Q units of electricity are transmitted 
against a force H, work EQ isdone ; also when they are trans- 
mitted up a difference of potential V'—V, work Q(V’—V) is 
done; but, in an open circuit containing an electromotive 
junction, V—V'is produced by and is equal to H. Hence, 
at an electromotive junction no work need be done by a cur- 
rent ; in other words, the existence or non-existence of a 
Peltier effect has nothing to do with the existence or non- 
existence of a local H.M.F. 
The fallacy of the argument, in either form, lies in over- 
precise specification of locality ; it gratuitously asserts as 
true for the junction what is only proved to be true for the 
whole circuit. It assumes that there can be no work done at 
a junction if it be perfectly easy to drive electricity either 
way across it—2. e. if there be no work done on the whole. 
11. To exhibit the fallacy, consider a hydrostatic analogy. 
Two vessels of water connected by a pipe in which is a motor 
of some kind, which, without leakage, exerts a specified force 
on the water and maintains a constant difference of potentials, 
but then remains stationary, doing no further work. We 
typify it feebly in the diagram by an impracticable close- 
fitting water-wheel driven by a weight without friction. 
V—V’ is the equivalent of the force exerted at the junction, 
and everything is in equilibrium. It is pertectly easy for 
water to flow from one vessel into the other under the influ- 
ence of the slightest extra force, for W helps the water up the 
hill V—V’, when the flow is in that direction ; and, when- 
ever the flow is reversed, it lets the water gently down again, 
taking all its energy out of it. If water is made to flow from 
A to B, say by pouring more into A, the weight W is lowered, 
or energy disappears (heat absorbed) at the junction ; if it is 
made to flow from B to A the weight is raised, or energy 
* Thus it may be, suggests Pellat, due to‘ a slight difference between 
E and —dV produced by the mere fact of a current passing ; 7. ¢. contact 
E.M.F. with electricity at rest may be slightly different from what it is 
with electricity in motion. 
