ation is into heat, and this has its mechanical
equivalent, but is not called mechanical energy, nor are the motions
which embody it similar. The mechanical ideas in these phenomena are
easy to grasp. They apply to the phenomena of the mechanics of large and
small bodies, to sound, to heat, and to light, as ordinarily considered,
but they have not been applied to electric phenomena, as they evidently
should be, unless it be held that such phenomena are not related to
ordinary phenomena, as the latter are to one another.

When we would give a complete explanation of the phenomena exhibited by,
say, a heated body, we need to inquire as to the antecedents of the
manifestation, and also its consequents. Where and how did it get its
heat? Where and how did it lose it? When we know every step of those
processes, we know all there is to learn about them. Let us undertake
the same thing for some electrical phenomena.

First, under what circumstances do electrical phenomena arise?

(1) _Mechanical_, as when two different kinds of matter are subject to
friction.

(2) _Thermal_, as when two substances in molecular contact are heated at
the junction.

(3) _Magnetic_, as when any conductor is in a changing magnetic field.

(4) _Chemical_, as when a metal is being dissolved in any solution.

(5) _Physiological_, as when a muscle contracts.

[Illustration: 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 fo