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
wire circuit.
Faraday's great discovery was, in fact, that when the pole of a magnet
is moved into, or moved out of, a coil of wire, the motion produces,
while it lasts, currents of electricity in the coil. Such currents are
known as "induced currents;" and the action is called magneto-electric
"induction.
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