Fig. 8.—Galvanic Battery.
(5) Physiological antecedents of electricity are exemplified by the structure and mode of operation of certain muscles (Fig. 9, a) in the torpedo and other electrical animals. The mechanical contraction of them results in an electrical excitation, and, if a proper circuit be provided, in an electric current. The energy of a muscle is derived from food, which is itself but a molecular compound loaded with energy of a kind available for muscular transformation. Bread-and-butter has more available energy, pound for pound, than has coal, and can be substituted for coal for running an engine. It is not used, because it costs so much more. There is nothing different, so far as the factors of energy go, between the food of an animal and the food of an engine. What becomes of the energy depends upon the kind of structure it acts on. It may be changed into translatory, and the whole body moves in one direction; or into molecular, and then appears as heat or electrical energy.
If one confines his attention to the only variable factor in the energy in all these cases, and traces out in each just what happens, he will have only motions of one sort or another, at one rate or another, and there is nothing mysterious which enters into the processes.
We will turn now to the mode in which electricity manifests itself, and what it can do. It may
be well to point out at the outset what has occasionally been stated, but which has not received the philosophical attention it deserves—namely, that electrical phenomena are reversible; that is, any kind of a physical process which is capable of producing electricity, electricity is itself able to produce. Thus to name a few: If mechanical
motion develops electricity, electricity will produce mechanical motion; the movement of a pith ball and an electric motor are examples. If chemical action can produce it, it will produce chemical action, as in the decomposition of water and electro-plating. As heat may be its antecedent, so will it produce heat. If magnetism be an antecedent factor, magnetism may be its product. What is called induction may give rise to it in an adjacent conductor, and, likewise, induction may be its effect.
Fig. 9.—Torpedo.