When the current which enters at B flows through the wire, the coil rotates, tending to set itself so that its north face is opposite the south face of the magnet.  If, when the helix has just reached this position, the current is reversed—­entering at B’ instead of B—­the poles of the coil are exchanged; the rotation, therefore, does not cease, but continues for another half turn.  Proper reversals of the current are accompanied by continuous motion, and since the disk and shaft rotate with the coil, there is continuous rotation.

If a wheel is attached to the rotating shaft, weights can be lifted, and if a belt is attached to the wheel, the motion of the rotating helix can be transferred to machinery for practical use.

The rotating coil is usually spoken of as the armature, and the large magnet as the field magnet.

310.  Mechanical Reversal of the Current. The Commutator.  It is not possible by hand to reverse the current with sufficient rapidity and precision to insure uninterrupted rotation; moreover, the physical exertion of such frequent reversals is considerable.  Hence, some mechanical device for periodically reversing the current is necessary, if the motor is to be of commercial value.

[Illustration:  FIG. 230.—­The commutator.]

The mechanical reversal of the current is accomplished by the use of the commutator, which is a metal ring split into halves, well insulated from each other and from the shaft.  To each half of this ring is attached one of the ends of the armature wire.  The brushes which carry the current are set on opposite sides of the ring and do not rotate.  As armature, commutator, and shaft rotate, the brushes connect first with one segment of the commutator and then with the other.  Since the circuit is arranged so that the current always enters the commutator through the brush B, the flow of the current into the coil is always through the segment in contact with B; but the segment in contact with B changes at every half turn of the coil, and hence the direction of the current through the coil changes periodically.  As a result the coil rotates continuously, and produces motion so long as current is supplied from without.

311.  The Practical Motor.  A motor constructed in accordance with Section 309 would be of little value in practical everyday affairs; its armature rotates too slowly and with too little force.  If a motor is to be of real service, its armature must rotate with sufficient strength to impart motion to the wheels of trolley cars and mills, to drive electric fans, and to set into activity many other forms of machinery.