I have been led to make these general allusions to electrical theory in order to emphasize the fact that in the present paper no unraveling of the mystery is to be attempted, but rather the presentation of some few considerations upon a subject of absorbing interest.

The conception of Faraday in regard to the existence of lines of magnetic force representing directions of magnetic strain or tension in a medium has not only lost nothing of its usefulness up to the present time, but has continually been of great service in the understanding of magnetic phenomena.  We need spend no time in showing, as Faraday and others have done, that these lines are always closed circuits, polarized so that the direction of the lines cannot be reversed without reversal of the actions.  Nor need we take time to show that in any medium the lines are mutually repellent laterally if of the same direction of polarization.  Opposing this tendency to separation or lateral diffusion of magnetic force is the strong apparent tendency of the lines to shorten themselves in any medium.  These actions are distributed by the presentation of a better medium, as iron instead of space or air.  Lines of force will move into the better medium, having apparently the constant tendency to diminish the resistance in their paths.

The peculiar and mysterious nature of media, such as iron, is to permit an extraordinary crowding of lines on account of slight resistance to their passage through it.  We need not, in addition, do more than refer to the other well-known facts of an electric current developing magnetic lines encircling the conductor, as being the general type, which includes all forms of magnetic field or electro-magnets, sustained by currents, and the fact of a development when magnetic lines or circuits and material masses are in relative movement of electromotive forces transversely to the direction of the lines of magnetism, and also transversely to the direction of relative movement, as in the case of electric conductors traversing or cutting through a field, or of a field traversing or being moved across a conductor.  We must not forget that even insulators, as well as conductors, cutting lines of force, have the electromotive force developed in them.  The action simply develops potential difference, and this generates the current where a circuit exists.  While we are in the habit of saying that a conductor moved across a field of lines, or vice versa, generates electric current, I think the statement incomplete.  The movement only sets up a potential difference, and the power expended in effecting the movement generates C x E. The current is energy less the potential, or the energy expended gives the two effects of potential or pressure and current or rate of movement.  Consequently an insulator, or an open-circuited conductor, traversing a field, consumes no energy, potential difference only being produced.  Nevertheless, as will be shown, the magnetic circuits or lines themselves may furnish the energy for their own movement across a conductor, and so develop current as well as potential.