Fig. 292.—Graphic representation of (1) an alternating current; (2) a pulsating current; (3) a continuous current.
Fig. 293.—DeLaval multi-stage turbine and gear driving 750-kw., 750-r.p.m., 600-volt direct-current generator.
A practical dynamo, however, has many coils upon its armature with a corresponding number of segments upon the commutator. (See Figs. 289 and 293.) As each coil and commutator segment passes a brush, it contributes an impulse to the current with the result that armatures with many coils produce currents that flow quite evenly. (See Fig. 292, 3.)
The current represented in Fig. 292 (2) is called a pulsating current.
Fig. 294.—A wire carrying a current across a magnetic field is pushed sideways by the field.
305. The electric motor is a machine which transforms the energy of an electric current into mechanical energy or motion. The direct current motor consists of the same essential parts as a direct current dynamo, viz., the field magnet, armature, commutator and brushes. Its operation is readily comprehended after one understands the following experiment:
Set up two bar electromagnets with unlike poles facing each other about an inch apart. A wire connected to a source of current is hung loosely between the poles as in Fig. 294. The circuit through the wire should contain a key or switch. If a current is sent through the electromagnets and then another is sent through the wire, the latter will be found to be pushed either up or down, while if the current is reversed through the wire it is pushed in the opposite direction. These results may be explained as follows:
Consider the magnetic field about a wire carrying a current (See Fig. 295.) If such a wire is placed in the magnetic field between two opposite poles of an electromagnet (Fig. 296), the wire will be moved either up or down. The reason for this is shown by the diagram in Fig. 297. Here a wire carrying a current and therefore surrounded by a magnetic field passes across another magnetic field. The two fields affect each other causing a crowding of the force lines either above or below the wire. The wire at once tends to move sideways across the field away from the crowded side. In the figure, the wire tends to move downward.