A magnetic needle held under a current-bearing wire turns on its pivot and finally comes to rest at an angle with the current.  The fact that the needle is deflected by the wire shows that the magnetic power of the wire extends into the surrounding medium.

The magnetic properties of current electricity were discovered by Oersted of Denmark less than a hundred years ago; but since that time practically all important electrical machinery has been based upon one or more of the magnetic properties of electricity.  The motors which drive our electric fans, our mills, and our trolley cars owe their existence entirely to the magnetic action of current electricity.

[Illustration:  FIG. 228.—­The coil turns in such a way that its north pole is opposite the south pole of the magnet.]

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 helix is capable of continued rotation around its support.

In practice, the rotating coil of a motor is arranged as shown in Figure 229.  Wires from the coil terminate on metal disks and are securely soldered there.  The coil and disks are supported by the strong and well-insulated rod R, which rests upon braces, but which nevertheless rotates freely with disks and coil.  The current flows to the coil through the thin metal strips called brushes, which rest lightly upon the disks.