another current is induced in the reverse direction to the first.
If the coil be closed through a small galvanometer G the movements
of the needle to one side or the other will indicate these
temporary currents. It follows from the principle of action and
reaction that if the magnet is kept still and the coil thrust over
it similar currents will be induced in the coil. All that is
necessary is for the wires to cut the lines of magnetic force
around the magnet, or, in other words, the lines of force in a
magnetic field We have seen already that a wire conveying a
current can move a magnetic pole, and we are therefore prepared to
find that a magnetic pole moved near a wire can excite a current
in it.

Figure 38 illustrates the conditions of this remarkable effect,
where N and S are two magnetic poles with lines of force between
them, and W is a wire crossing these lines at right angles, which
is the best position. If, now, this wire be moved so as to sink
bodily through the paper away from the reader, an electric current
flowing in the direction of the arrow will be induced in it. If,
on the contrary, the wire be moved across the lines of force
towards the reader, the induced current will flow oppositely to
the arrow. Moreover, if the poles of the magnet N and S exchange
places, the directions of the induced currents will also be
reversed. This is the fundamental principle of the well known
dynamo-electric machine, popularly called a dynamo.

Again, if we send a current from some external source through the
wire in the direction of the arrow, the wire will move OF ITSELF
across the lines of force away from the reader, that is to say, in
the direction it would need to be moved in order to excite such a