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.

The strength of a motor may be increased by replacing the singly
coiled armature by one closely wound on an iron core; in some