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.
This occurs in the effort of lines to shorten their paths, to lessen
their density, to pass to better media. Indeed, a close examination
will show that wherever power is expended in developing current in a
circuit, cutting lines of force, the energy expended is first employed
in stretching the lines, which thus receive the energy required to
permit them, in shortening, to cut the conductor and set up currents
in the electric circuit in accordance with the potential difference
developed in that circuit and its resistance.
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