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Page:Popular Science Monthly Volume 56.djvu/335

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WHAT MAKES THE TROLLEY CAR GO.

magnet, and the rotating part is called the armature, while the device by means of which the direction of the current through the armature coil is reversed is called the commutator. In this last figure it will be noticed that the coils wound upon the field magnet are represented as of wire much finer than that wound upon the armature. In actual practice machines are sometimes wound in this way, and sometimes the field wire is twice as large as that on the armature. When the field wire is very much finer than that of the armature the machine is what is known as shunt wound, which means that only a small portion of the current that passed through the armature passes through the field coils. Although with this type of winding the current that passes through the field coils is very weak, the magnetism developed thereby can be made greater than that of the armature if desired. This result is accomplished by increasing the number of turns of wire in the field coils. Thus if the current through the armature is one hundred times as strong as that through the field coils, the latter can be made to equal the effect of the former by increasing the number of turns in the proportion of one hundred to one, and if the increase is still greater the field coils will develop the strongest magnetism. The reason why a small current passing around a magnet a great many times Fig. 9. Fig. 10.

PSM V56 D0335 Diagram of the electric motor principle.png
Fig. 9. Fig. 10
Fig. 9. 10.—Diagrams illustrating the Principles of the Electric Motor.

will develop as strong a magnetization as a large current, can be readily understood when we say that the magnetism is in proportion to the total strength of the electric current that circulates around the magnet. Suppose we have two currents, one of which is one thousand times as strong as the other, then if the weak one is passed through a coil consisting of one thousand turns it will develop just as strong a magnetization as the large current passing