An electric motor (Figs. 88, 89, 90, 91) is a machine for transforming electrical into mechanical energy. An electric current causes the armature to rotate, and the mechanical energy due to the rotation may be utilized to drive machinery. The motor is quite similar to a dynamo; in fact, the direct current motor is almost identical with the dynamo in structure and circuit, although in detail of design its external appearance is sometimes quite different. The principle of magnetism on which the direct current motor works is as follows:
Fig. 88. - An Electric Motor.
When a current of electricity is passed through a coil of wire on the armature, the coil will always revolve so as to include as many lines of force as possible. When it reaches this position, the commutator changes the current in the coil so that the armature must again rotate a half-revolution in order to include the greatest number of lines of force. Each turn of wire acts in the same way, so that the continual force acting on the armature causes it to rotate. By means of shaft and pulley, the energy may be transmitted to other machines and made to do work. The direction in which a motor runs may be reversed by changing the connection so that the direction of current is reversed through either field or armature.
Fig. 89. - Brush-Holders.
Fig. 90. - Poles of a Multi-Pole Dynamo.