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
armatures there are thousands of turns of wire. The presence of soft
iron within the armature (Section 296) causes greater attraction
between the armature and the outside magnet, and hence greater force
of motion. The magnetic strength of the field magnet influences
greatly the speed of the armature; the stronger the field magnet the
greater the motion, so electricians make every effort to strengthen
their field magnets. The strongest known magnets are electromagnets,
which, as we have seen, are merely coils of wire wound on an iron
core. For this reason, the field magnet is usually an electromagnet.

When very powerful motors are necessary, the field magnet is so
arranged that it has four or more poles instead of two; the armature
likewise consists of several portions, and even the commutator may be
very complex. But no matter how complex these various parts may seem
to be, the principle is always that stated in Section 309, and the
parts are limited to field magnet, commutator, and armature.

[Illustration: FIG. 231.--A modern power plant.]

[Illustration: FIG. 232.--The electric street car.]

The motor is of value because by means of it motion, or mechanical
energy, is obtained from an electric current. Nearly all electric
street cars (Fig. 232), are set in motion by powerful motors placed
under the cars. As the armature rotates, its motion is communicated by
gears to the wheels, the necessary current reaching the motor through
the overhead wires. Small motors may be used to great advantage in the
home, where they serve to turn the wheels of sewing machines, and to
operate washing machines. Vacuum cleaners are frequently run by
motors.

CHAPTER XXXIV

HOW ELECTRICITY MAY BE MEASURED

312. Danger of an Oversupply of Current. If a small toy motor is
connected with one cell, it rotates slowly; if connected with two
cells, it rotates more rapidly, and in general, the greater the number
of cells used, the stronger will be the action of the motor. But it is
possible to send too strong a current through our wire, thereby
interfering with all motion and destroying the motor. We have seen in
Section 288 that t