ly upon the disks.

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