Then another famous discoverer found that by winding a wire round a bar of iron and sending a current of electricity through the wire he turned the bar of iron into a magnet. As long as the current was passing through the wire the iron bar acted just like a permanent steel magnet; it would attract pieces of iron and hold up nails, but the moment the current was stopped the bar lost its magnetism, the nails or pieces of iron dropped off, and it became just an ordinary bar of iron again. This invention is called the electro-magnet, and the electro-magnet is used in some form or other in every electrical industry.
The electric dynamo owes its being to those two discoveries. It consists of coils of copper wire wound on a shaft, and that shaft is revolved close to a powerful magnet. The influence of the magnet causes electric currents to be produced in the coils of copper wire, and these currents are delivered by the coils into suitable conductors or wires by means of which the currents are led to the place where they have to do their work. One very interesting thing about the modern dynamo machine is that it is what electrical men call "self-exciting." That does not mean that it gets into a state of excitement about itself. It means that the dynamo provides its own magnetism. At first dynamos were made with great big steel magnets, but those were very expensive and unsatisfactory. Then a clever inventor hit upon the plan of using electro-magnets, and sending part of the current of the dynamo through their coils to give them magnetism. This is the action of the self-exciting dynamo. When the collection of coils wound on the revolving shaft first begins to turn, very little current is produced, because there is very little magnetism in the iron magnets. Part of this current goes through the magnet coils and increases the magnetism; this strengthens the current in the coils, and this process goes on until, after a few minutes, the magnets are fully magnetized and the coils are giving their full strength of current.
[Illustration: Diagram showing the electric circuit through each car, and illustrating the method of sending more than one car along the same wire.]
Some time after this was discovered, it became known that if the two wires from a dynamo were joined to a second dynamo instead of to an electric lamp, this second dynamo would revolve, and could be used to drive a machine, such as a sewing-machine or a printing-press.