[Illustration: FIG. 3.]
Let N S, Fig. 3, be a bar of hardened steel rendered permanently
magnetic. If now there be brought near to it a board-nail, the latter
will become a magnet through the _inductive_ action of the first magnet.
This induced magnetism may be demonstrated by bringing a tack or other
bit of iron to the end that is farthest from the permanent magnet; the
tack will adhere to the nail, but will fall off when the nail is removed
from the neighborhood of the magnet. By testing the polarity of the
nail, it will be found that the end nearest the magnet will be a south
pole if the magnet has its north pole towards it, in all cases having a
polarity opposite to that of the pole acting upon it. The strength of
this induced magnetism thus developed depends upon the distance apart of
the magnet and the iron, being at its maximum when the two touch. But
the tack itself is also made a magnet, and will attract another tack,
and that one still another, the number which can be thus supported being
dependent upon the strength of the first or inducing magnet.
Suppose now that we should wind a few feet of wire about the nail, and
fasten the two ends of the wire to an ordinary galvanometer, and then
make the nail to approach the permanent magnet. The galvanometer needle
would be seen to move as the nail approached; and, if the latter were
allowed to touch the magnet, the movement of the needle would suddenly
be much hastened, but would directly come to rest, showing that, so
long as there is no motion of the nail towards or away from the magnet,
no electricity is moving in the wire, although the nail is a strong
magnet while it is in contact with the permanent magnet. If the nail be
now withdrawn, the two phenomena happen as before: that is to say, as
the nail recedes it loses its magnetism; and the giving-up of its
magnetism induces a current of electricity through the wire in the
opposite direction to that it had when the nail approached. The current
of electricity in the opposite direction is indicated by the
galvanometer needle, which moves according to Ampere's law mentioned on
a preceding page.
It may be noted here that we have an effect quite analogous to that
already mentioned on page 21 as the experiment of Faraday. In one case a
permanent magnet is thrust into a coil of wire, and in the other a piece
of iron is made a magnet while enclosed in a coil. In each case there is
generated a current of electricity _which lasts no longer than the
mechanical motion of the parts lasts_.