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,
unless we are to regard the lines as simply indicating a sort of
atmosphere of magnetic effect whose density becomes less as we proceed
outward from the wire, in which case either form of symbol suffices.
The direction of polarization of the lines may be indicated by an
arrow head pointing in a direction of right-handed rotation in the
path of the lines. This is the typical figure or expression for all
forms of simple magnetic circuit--the form of the lines, their length,
position, density, will depend on the shape of the conductor or
conductors (when more than one) and the materials surrounding or in
proximity to the wire or wires.
If the current traversing the conductor is constant, the magnetic
field around it is stable and static, unless other influences come in
to modify it. The cutting off of the current is followed by
instability of the field whereby it can and must produce dynamic
effects. I say _must_ because the field represents stored energy, and
in disappearing _must_ give out that energy. To throw light on this
part of the subject is one of the objects of the present paper.
Cutting off the current supply in the case assumed leaves the
developed magnetic lines or strains unsupported.
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