[Illustration: FIG. 52--ONE OF EDISON'S FIRST DYNAMOS Permission of Association of Edison Illuminating Companies.] [Illustration: FIG. 53--A DYNAMO MOUNTED ON THE TRUCK OF A RAILWAY CAR The dynamo furnishes current for the electric lights in the car. When the train is not running the current is furnished by a storage battery.] Electric Power It has been said that the nineteenth century was the age of steam, but the twentieth will be the age of electricity. Before the end of the nineteenth century, however, electric power had become a reality, and there remained only development along practical lines. We must turn to Oersted, Ampère, and Faraday to find the beginning of electric power. In Oersted's experiment, motion of a magnet was produced by an electric current. Ampère found that electric currents attract or repel each other, and this because of their magnetic action. Faraday found that one pole of a magnet will spin round a wire through which a current is flowing. Here was motion produced by an electric current. These great scientists discovered the principles that were applied later by inventors in the electric motor. A number of motors were invented in the early years of the century, but they were of no practical use. It was not until after the invention of the Gramme and Siemens dynamos that a practical motor was possible. It was found that one of these dynamos would run as a motor if a current were sent through the coils of the armature and the field-magnet; in fact, the current from one dynamo may be made to run another similar machine as a motor. Thus the dynamo is said to be reversible. If the armature is turned by a steam-engine or some other power, a current is produced. If a current is sent through the coils, the armature turns and does work. If the machine is used to supply an electric current, it is a dynamo. If used to do work--as, for example, to propel a street-car and for that purpose receives a current--it is a motor. The same machine may be used for either purpose. In practice there are some differences in the winding of the coils of dynamos and motors, yet any dynamo can be used as a motor and any motor can be used as a dynamo. This discovery made it possible to transmit power to a distance with little waste as well as to divide the power easily. The current from one large dynamo may light streets and houses, and at the same time run a number of motors in factories or street-cars at great distances apart. A central-station dynamo may run the motors that propel hundreds of street-cars. Dynamos at Niagara furnish current for motors in Buffalo and other cities. One great scientist, who no doubt fore-saw the wonders of electricity which we know so well to-day, said that the greatest discovery of the nineteenth century was that the Gramme machine is reversible. The First Electric Railway