This experimental evidence must suffice to establish two of the three
fundamental points stated at the outset, for they prove conclusively
that the electric current may be treated as a magnetic phenomenon, and
that both in the case of the pole of a magnet, and in that of the wire
which carries a current, a portion, at any rate, of the energy of the
magnetic forces exists outside the magnet or the current, and must be
sought in the surrounding space.

[Illustration: Fig. 5]

[Illustration: Fig. 6]

Having grasped these two points, the next step in our argument is to
establish the relation between the current and the magnet, and to show
how one may produce the other.

[Illustration: Fig. 7]

If we wind a piece of copper wire into a helix or spiral, as in Fig. 7,
and pass a current of electricity through it, the magnetic whirls in the
surrounding space are modified, and the lines of force are no longer
small circles wrapping round the conducting wire. For now the lines of
force of adjacent strands of the coil merge into one another, and run
continuously through the helix from one end to the other. Compare this
figure with Fig. 1, and the similarity in the arrangement of the lines
of force is obvious. The front end of the helix acts, in fact, like the
north pole of a magnet, and the further end like the south pole. If a
small bar of iron be now pushed into the interior of this helix, the
lines of force will run through it and magnetize it, converting it into
an _electro-magnet_. The magnetic "field" of such an electro-magnet is