reaks
in the continuity around the center of the conductor. The total
magnetism which could be set up by a current would be lessened, and
the self-induction, therefore, lessened.

[Illustration: Fig. 2.]

[Illustration: Fig. 3.]

The moment we begin the bringing of iron into proximity with an
electric conductor conveying current, we provide a better medium for
the flow or development of magnetic lines or circuits. In other words,
the lines may then be longer, yet equally intense, or more lines may
be crowded into a section of this metal than in air or space. Figs.
4a, 4b, 4c show the effect brought about by bringing iron of
different forms near to the conductor.

[Illustration: Fig. 4a.]

[Illustration: Fig. 4b.]

[Illustration: Fig. 4c.]

It shows, in other words, the development of the ordinary
electro-magnet of the horseshoe form, and the concentration of the
lines in the better medium. The lines also tend to shorten and
diminish the resistance to their passage, so that attraction of the
iron to the conductor takes place, and if there is more than one piece
of iron, they tend to string themselves around the conductor in
magnetic contact with one another.

When copper bars of 1 inch diameter are traversed by currents of
40,000 to 60,000 amperes, as in welding them, the magnetic forces just
referred to become so enormous that very heavy masses of iron brought
up to the bar are firmly held, even though the current be of an
alternating character, changing direction many times a second.

[Illustration: Fig. 5]

[Illustration: Fig. 6]

When a conductor is surrounded by a cast iron ring, as in Fig. 5, the
current in such conductor has an excellent magnetic medium surrounding
it. A large amount of energy is then abstracted on the first impulse
of current, which goes to develop strong and dense magnetic lines
through the iron ring and across the gap in it. On taking off the
current the energy is returned as extra current, and its force is many
times what would be found with air alone surrounding the conductor. We
have then greatly increased the self-induction, the storing of energy
and opposition to current flow at the beginning, the giving back of
energy and assistance to the current flow on attempting to remove or
stop the current. Let us now complete the ring, by making it of iron,
endless, Fig. 6, with the conductor in the middle.

We now find that on passing current through the conductor it meets
with a