pposed in effect
everywhere by the opposite current in its neighbor.

Where a number of conductors are parallel, and have the same direction
of current, as in a coil or in a strand, it is evident that statically
the conductor may be considered as replaceable by a single conductor
with the same external dimensions and same total current in the area
occupied, the magnetic forces or lines surrounding them being of same
intensity. But with changing current strength the distribution of
current in the conductor has also a powerful effect on the energy
absorbed or given out in accordance with the magnetism produced. Hence
the self-induction of a strand, coil or conductor of the same section
varies with the rapidity of current changes, owing to the conduction
being uneven.

The uneven distribution of current, or its tendency to flow on the
outer parts of a conductor when the rate of variation or alternation
is made great, is in itself a consequence of the fact that less energy
is transferred into magnetism in this case than when the current flows
uniformly over the section, or is concentrated at the center. In other
words, when a uniform current traverses a conductor of the same
section, the circular magnetism, or surrounding magnetic lines, are to
be found not only outside the conductor, but also beneath its
exterior. Since in forming these lines on passage of current the
middle of section would be surrounded by more lines than any other
part of the conductor, the current tends to keep out of that part and
move nearer the exterior in greater amount. Hence, in rapidly
alternating currents the conductor section is practically lessened,
being restricted largely to the outer metal of the conductor. If the
round conductor, Fig. 2, were made of iron, the magnetism interior to
it and set up by a current in it would be very much greater, the
section of the conductor being filled with magnetic circuits or lines
around the center. The total magnetism, external and internal, would
be much greater in this case for a given current flow, and the energy
absorbed and given out in formation and loss of field or the
self-induction would be much increased. This could, however, be
greatly diminished by slitting the conductor radially or making it of
a number of separate wires out of lateral magnetic contact one with
the other, Fig. 3. In these cases the resistance of the interior
magnetic circuits would be increased, as there would be several b