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