118 
Lord Kelvin on 
[b.s.e., sess. 
On the Thermodynamics of Volta-contact Electricity. 
By the Rt. Hon. Lord Kelvin, G.C.V.O., etcx 
(Read February 21, 1898.) 
§ 1. Let X and Y he two metals, of which X is electrically 
positive to Y in the Volta-contact series for dry metals, and make 
an incomplete circuit of them as indicated in the diagram, with X 
and Y metallically connected at an interface J, and free surfaces 
II, KCC exposed, with ether or air, or any gas or insulating fluid, 
between them. Let CC he a movable slab of the Y-metal, resting 
frictionlessly on a fixed surface KK of the same metal. If left to 
itself the movable slab would, in virtue of electric attraction, as we 
shall see, oscillate on the two sides of the middle position in which 
the whole of its upper surface is opposite to II. We shall suppose 
it held by applied force F in any position, or moved, or allowed to 
move, from any one position to any other, at our pleasure. 
§ 2. Suppose now our apparatus to be given with no excess of 
either electricity above the other, and to be insulated in air or 
ether at a distance from all other bodies great in comparison with 
its own dimensions, and with no electrified body near enough to it 
to produce sensible electrification through influence. Every part 
of the X-surface will be found positively and every part of the 
Y-surface negatively electrified, provided each surface is of per- 
fectly uniform Volta-quality throughout its extent; but the electric 
surface-densities of the opposite electrifications will be everywhere 
exceedingly small, except in and near the portions of II and CC 
closely opposed to one another. Hence, if the slab is drawn out- 
wards, an electric current will flow from II through the X-metal, 
and will cross the junction J from X to Y and flow through Y to 
compensate negative electricity on the portions of CC passing from 
close opposition to II. Our thermodynamic operations, of which 
we will arrange a Carnot cycle, are drawing in and out the slab 
CC, sometimes with the whole metal coated with an ideal varnish 
impermeable to heat, and sometimes with the whole surface kept 
