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.

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