31.
In explanation of the fact that the magneto-electric currents are of a certain duration, great caution must be taken not to adopt that hypothesis which most easily presents itself; for it is very rare that such an explanation is correct, or even most probable. We shall certainly conclude from it, that some time is required for the soft iron to attain the maximum of magnetic force, or for the development of the magnetism to become any function of the time. The mathematical idea of a force requires that it have an existence independent of time. This is the case with celestial attraction; as soon as a planet appears opposite to the sun, the force of attraction, in conformity with its actual position, takes place according to the Newtonian law; no time is necessary for the entire mass of the body to become seized by gravitation. Here is the type of a force which serves as a basis, if we wish to form a general and correct idea of it. But when a force acts on any system of material points, and if we substitute in place of the primitive effects, the active force this system has acquired, this latter force will be a function of the time during which the action of the force was in operation. This is incontestable, but I think it would not be possible to hazard the inverse conclusion; for instance, where we perceive the development or the propagation of a force requires time, it is a molecular movement, or the movement of any matter taking place. Thus, if we suppose that the development of the magnetism be not instantaneous, we must admit, either that the magnetism consists in the movement of a fluid, or of some æther, or that the magnetic force produces some molecular movement, or positive displacement of the particles of the soft iron. I am of the latter opinion; the time required for the propagation of magnetism being too considerable to allow of attributing this force to the movement of an æther, analogous to the luminous or the electric æther, the rapidity of which, according to Mr. Wheatstone's experiments, surpasses even that of light. There is nothing to oppose this, in the supposition of a positive contraction taking place in the soft iron as soon as it is exposed to magnetic influence. The armature which is attracted by a force equivalent to a weight of 1000 lb., would exercise the same mechanical effects as a weight of the same magnitude attached to an armature of a non-magnetized horse-shoe; and it is not needful to employ very delicate instruments to estimate the compression produced by such a charge.
