309. The Principle of the Motor. If a close coil of wire is
suspended between the poles of a strong horseshoe magnet, it will not
assume any characteristic position but will remain wherever placed.
If, however, a current is sent through the wire, the coil faces about
and assumes a definite position. This is because a coil, carrying a
current, is equivalent to a magnet with a north and south face; and,
in accordance with the magnetic laws, tends to move until its north
face is opposite the south pole of the horseshoe magnet, and its south
face opposite the north pole of the magnet. If, when the coil is at
rest in this position, the current is reversed, so that the north pole
of the coil becomes a south pole and the former south pole becomes a
north pole, the result is that like poles of coil and magnet face each
other. But since like poles repel each other, the coil will move, and
will rotate until its new north pole is opposite to the south pole of
the magnet and its new south pole is opposite the north pole. By
sending a strong current through the coil, the helix is made to rotate
through a half turn; by reversing the current when the coil is at the
half turn, the helix is made to continue its rotation and to swing
through a whole turn. If the current could be repeatedly reversed just
as the helix completed its half turn, the motion could be prolonged;
periodic current reversal would produce continuous rotation. This is
the principle of the motor.

[Illustration: FIG. 229.--Principle of the motor.]

It is easy to see that long-continued rotation would be impossible in
the arrangement of Figure 228, since the twisting of the suspending
wire would interfere with free motion. If the motor is to be used for
continuous motion, some device must be employed by means of which the