Fig. 5.—Frictional electrical machine.
Each of these has several varieties, and changes may be rung on combinations of them, as when mechanical and magnetic conditions interact.
(1) In the first case, ordinary mechanical or translational energy is spent as friction, an amount measurable in foot-pounds, and the factors we
know, a pressure into a distance. If the surface be of the same kind of molecules, the whole energy is spent as heat, and is presently radiated away. If the surfaces are of unlike molecules, the product is a compound one, part heat, part electrical. What we have turned into the machine we know to be a particular mode of motion. We have not changed the amount of matter involved; indeed, we assume, without specifying and without controversy, that matter is itself indestructible, and the product, whether it be of one kind or another, can only be some form of motion. Whether we can describe it or not is immaterial; but if we agree that heat
is vibratory molecular motion, and there be any other kind of a product than heat, it too must also be some other form of motion. So if one is to form a conception of the mechanical origin of electricity, this is the only one he can have—transformed motion.
Fig. 6.—Thermo-pile.