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.

I think we may also say, though I do not remember to have seen the
statement so put, that whenever electric potential is set up
inductively, as in self-induction, mutual induction, induction from
one circuit to another, and induction from magnets or magnetic field,
it is set up by the movement of lines of force laterally across the
body, mass or conductor in which the potential is developed, and that
whenever current is set up in a wire or an existing current prolonged,
or an existing current checked by induction, self-induction, or
induction from magnets, the action is a transfer of energy,
represented by strained lines of force shortening or lessening their
resistance, or lengthening and increasing the resistance in their
paths. The magnetic field is like an elastic spring--it can in one
condition represent stored energy--it can be strained and will store
energy--it can be made to relieve its strain and impart energy.

[Illustration: Fig. 1.]

Let us examine some known phenomena in this light. Take the case of a
simple wire, conveying current, say, in a line away from observer,
Fig. 1. There exists a free field of circular magnetism (so called),
shading off away from the wire, and which is represented by concentric
circles of increased diameter. The superior intensity or strength of
the lines near the wire may also be represented by their thickness.
This is often shown also by crowding the lines near the wire, though I
am disposed to regard Fig. 1 as more nearly expressing the condition,