circuit the pole, N, is attracted, the tendency being to draw as many
lines of force as possible into the embrace of the circuit.

[Illustration: Fig. 11.]

So far as the reasoning about these mutual actions of magnets and
currents is concerned, it would therefore appear that the lines of force
are the really important feature to be understood and studied. All our
reasons about the attractions of magnets could be equally well thought
out if there were no corporeal magnets there at all, only collections
of lines of force. Bars of iron and steel may be regarded as convenient
conductors of the lines of force; and the poles of magnets are simply
the places where the lines of force run out of the metal into the air or
_vice versa_. Electric currents also may be reasoned about, and their
magnetic actions foretold quite irrespective of the copper wire that
acts as a conductor; for here there are not even any poles; the lines
of force or magnetic whirls are wholly outside the metal. There is an
important difference, however, to be observed between the case of the
lines of force of the current, and that of the lines of force of the
magnet. The lines of force of the magnet are the magnet so far as
magnetic forces are concerned; for a piece of soft iron laid along the
lines of force thereby becomes a magnet and remains a magnet as long
as the lines of force pass through it. But the lines of force crossing
through a circuit are not the same thing as the current of electricity
that flows round the circuit. You may take a I loop of wire and put the
poles of magnets on each side of it so that the lines of force pass
through in great numbers from one face to the other, but if you have
them there even for months and years the mere presence of these lines
of force will not create an electric current even of the feeblest kind.
There must be _motion_ to induce a current of electricity to flow in a