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Page:Popular Science Monthly Volume 56.djvu/427

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413
WHAT MAKES THE TROLLEY CAR GO.

as belonging to the series parallel type, the name being given from the fact that when the car is running slow, the current passes through the two motors in series—that is, through one after the other; but when the motors are running fast, a separate current passes through each machine.

If, when a car is running, the controlling switch is turned to cut the current off, the effect will be that the speed will gradually

PSM V56 D0427 View of electric railway truck and magnetic brakes.png
Fig. 20.—View of Electric Railway Truck with One Motor on each Axle and provided with Magnetic Brakes.

reduce; but if it is desired to effect a sudden stop, it becomes necessary to cheek the headway by means of a brake. For this purpose the hand brake ordinarily used on all types of cars is employed, but magnetic brakes are also used in some cases. Fig. 20 shows a car truck equipped with two motors and magnetic brakes, one on each axle. Looking at the front end of the truck, the brake is seen on the left side of the axle, between the motor bearing and the car wheel. The larger drum, on the right side, is the casing within which the gear wheel and pinion are inclosed. These magnetic brakes are operated by a current generated by the motors, and not by that of the main line. As was explained in the first article, an electric motor can be made to act as a generator of electric current by simply reversing the direction in which the armature revolves. If we do not desire to reverse the direction of rotation, the result can still be attained by reversing the direction in which