ence the magnetic field represents work or potential
energy. But to develop potential in the wire the lines must cut the
wire. This they can do by collapsing or closing on themselves. The bar
seems, therefore, to lose its magnetism by gaining it all, and in
doing so all the external lines of force moving inward cut the wire.
The magnetic circuits shorten and short-circuit themselves in the bar,
perhaps as innumerable molecular magnetic circuits interior to the
iron medium. To remagnetize the bar we may pass an electric current
through the coil. The small closed circuits are again distended, the
free field appears, and the lines moving outward cut across the wire
coil opposite to the former direction and produce a counter potential
in the wire, and consequent absorption of the energy represented in
the free field produced. As before studied, the magnetism cannot
disappear without giving out the energy it represents, even though the
wire coil be on open circuit, and therefore unable to discharge that
energy. The coil open-circuited is static, not dynamic. In such
assumed case the lines in closing cut the core and heat it. Let us,
however, laminate the core or subdivide it as far as possible, and we
appear to have cut off this escape for the energy. This is not really
so, however. We have simply increased the possible rate of speed of
closure, or movement of the lines, and so have increased for the
divided core the intensity of the actions of magnetic friction and
local currents in the core, the latter still receiving the energy of
the magnetic circuit. This reasoning is based on the possibility in
this case of cutting off the current in the magnetizing coil and
retaining the magnetic field. This is of itself probably impossible
with soft iron. That the core receives the energy when the coil cannot
is shown in the well known fact that in some dynamos with armatures of
bobbins on iron cores, the running of the armature coils on open
circuit gives rise to dangerous heating of the cores, and that under
normal work the heating is less. In the former case the core
accumulates the energy represented in the magnetic changes. In the
latter the external circuit of the machine and its wire coils take the
larger part of the energy which is expended in doing the work in the
circuit. In this case, also, the current in the coils causes a
retardation of the speed of change and extent of change of magnetism
in the iron cores, which keeps d