Open main menu

Page:Popular Science Monthly Volume 24.djvu/765

This page has been proofread, but needs to be validated.

conservation of energy is afforded by the fact that a dynamo will not only generate an electric current if it be revolved by mechanical means, but that it will itself revolve, if an electric current be sent through it from an exterior source; so that it not only can transform mechanical energy into electrical energy, but can also transform electrical into mechanical energy. "When used for this purpose it is called an "electro-motor," and sometimes an "electric engine."

Not only, however, is it necessary for an engine to be capable of doing a certain kind of work; it is also necessary for it to be capable of doing it economically, and it is for this reason that such a great future is prophesied for electric engines. For, while an excellent and elaborately constructed stationary steam-engine can produce but a small fraction of the energy it absorbs, a good electric engine (or electro-motor) will return seventy-five per cent of the electric energy given it by the generating dynamo. For the reason, however, that no economical means of generating large currents are yet discovered, except the method described of first burning coal, the use of electric machinery is at present restricted to certain industries. Now, one of these industries is believed to be railroading.

The opinion is generally held that railroad companies desire to obtain as large a return as possible upon their investment, and therefore to run their trains as cheaply as possible. If this be true, the value of an electric railway will become obvious, when one remembers that, of necessity, the present locomotive is wasteful in the extreme, and that in an electric railway a large and economical stationary engine renders its mechanical energy to a large and economical dynamo which sends an electric current to an economical motor on an electric locomotive. This motor is connected with the driving-wheels by gearing, belting, or other suitable devices, so that its revolution produces a revolution of the driving-wheels and a consequent progressive motion of the electric locomotive, in the same way that the engine of a steam-locomotive produces a rotary motion of the driving-wheels, and a consequent progressive motion of the steam-locomotive. There is a certain loss of electricity in passing from the dynamo to the motor on the locomotive, both from leakage and from overcoming the resistance of the conductors; but, for distances not too great, this loss, added to the losses in converting the mechanical energy of the stationary engine into electrical energy, and in reconverting this electrical energy back into mechanical energy by the motor, is not equal to the loss inseparable from even the best steam-locomotives.

It will be, of course, noticed that it is necessary constantly to maintain an electrical connection between the electro-motor on the locomotive and the stationary dynamo, in all positions of the locomotive. To accomplish this effectively, a number of systems have been invented. By one system the rails themselves act as conductors, the