Harper's Electricity Book for Boys

by Joseph H. Adams

The use of the electric motor to propel vehicles of all kinds is called electric traction. It is, of course, a branch of electric power, which we have just been considering; and it is in many respects the most important branch. The wealth of a country is largely built up and maintained by its facilities for transportation, such as its canals, highways, railroads, and street and interurban car-lines.

In this field electric power is playing a most important part, although it was not many years ago that the first experimental electric cars were put in to replace horses on the street-railways of our cities. The change was found to be so successful that the field of the trolley-car was widened and extended very rapidly, until now we have our great suburban and interurban electric railways, with cars almost or quite as big as those on the steam-railroads and running at even higher speeds. During the last few years, also, the sphere of the steam-railroad itself has been invaded by electricity, by the construction of powerful electric locomotives to draw passenger and freight trains.

The Trolley-car

Let us consider just what it is that makes a trolley-car go. Since electric power is only mechanical energy in another form, we know that the motionless copper trolley-wire, suspended over the track in our streets, is the means of propelling the car just as truly—though in a different way—as if it were a moving steel cable to which the car was attached. We must keep in mind the fact that the electricity is not itself the source of power, but only the medium of transmission. The engine in the power-house, by turning a dynamo there, maintains a constant electric pressure, or “constant potential,” as it is termed, in the trolley-wire. This pressure of electricity forces the power through the motors of the car as soon as the motorman makes the connection to them by turning the handle of his “controller.”

Fig. 11

The Continuous-current Motor

[Fig. 11] is a view of one form of continuous-current motor. There is not much of the motor itself to be seen, because it is entirely enclosed in a cast-iron case. The shaft of the motor has a small “spur gear” fixed on one end, driving a gear-wheel which is fixed on the car axle. By this arrangement more than one revolution of the motor armature is required to make one revolution of the car-wheel, which multiplies the force exerted in turning the wheel.

Fig. 12