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 the amount of current which can safely flow through a wire depends upon the thickness of the wire.  A strong current sent through a fine wire has its electrical energy transformed largely into heat; and if the current is very strong, the heat developed may be sufficient to burn off the insulation and melt the wire itself.  This is true not only of motors, but of all electric machinery in which there are current-bearing wires.  The current should not be greater than the wires can carry, otherwise too much heat will be developed and damage will be done to instruments and surroundings.