§ 1. Let X and Y be 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 be a movable slab of the Y-metal, resting frictionlessly on a fixed surface KK of the game 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 perfectly 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.