An electric engine includes two main parts: the dynamo, which produces or generates the electricity, and the motor, through which the electricity is converted into power. Before an electric engine could be constructed, it was necessary to discover a method of producing electricity in great quantities at small cost, or to invent the dynamo; and also to find a way to change, with small loss, electricity into power, or to invent the motor. Besides, a great deal had to be learned about electricity itself. The electric engine, like most inventions, is therefore not the work of one man, but of many men working at different times and in different countries.

Almost everybody knows a little about electricity. Very often on a cold day, if one rubs his feet on a carpet and then touches another person, a crackling sound will be heard, and the person touched will receive a shock. Something like this happens when a cat's back is rubbed briskly. Despite these and other interesting things that have long been known, not much interest was taken in electricity so that it was not until very recently that much was really understood about it. Men did not begin to study electricity with care until about two hundred years before Washington became President.

Otto von Guericke, a German, was one of the first to study it carefully. He made the first machine to generate or produce electricity. His machine was merely a globe of sulphur supported so that he could turn it by a crank. When he placed his dry hand on the moving globe, it would attract bits of paper like a magnet. A similar machine was made later by placing a glass disk so that it could be easily turned, and by fixing a number of rubber or silk brushes so that they would rub against the revolving glass.

Alexander Volta, an Italian, was also a pioneer. He discovered in 1800 that two different metals in contact with each other would produce an electric current. From this discovery, he invented an electric battery. It consisted of a number of cups piled on top of each other. In each cup he placed a disk of copper and a disk of zinc, covered with a brine of common table salt. The copper disk of the first cup was connected by a copper wire with the zinc disk of the second cup, and so on. A copper wire was also fastened to the copper disk of the first cup and one to the zinc disk of the last cup. On taking hold of these last two connecting wires a strong electric shock was felt, and the current continued to flow regularly. A battery like Volta's can be made without the cups by using a glass jar. It should be remembered, however, that the strength of the battery does not depend upon the size, but upon the number of the disks or plates.

Volta's battery was the first easy way found to produce electricity in quantities. Years of study and experiment have shown also that the metals used by Volta, copper and zinc, are the very best to employ in batteries. The so-called "dry battery," for example, used to work doorbells, was made until lately of copper and zinc disks covered with sand or sawdust, soaked in acid and sealed. The battery is to-day the most common of electrical appliances. There are one or two in almost every home.

Most boys at some time or other have owned a magnet, which, as you know, will pick up or attract bits of paper, pins, or even filings. To make a compass, stroke a needle from end to end with one end of a magnet, and float the needle on a bit of cork. Iron filings arrange themselves in file like live soldiers, if a magnet is placed under a sheet of paper and the filings are spilled over the paper. It is the invisible currents of magnetism flowing around the magnet that make the filings squirm about.

For many hundreds of years, bar and horseshoe magnets were made by rubbing small iron bars over a natural magnet or loadstone. Loadstones are pieces of hard, black rock found in Asia Minor, China, and Japan. They were thought to confer peculiar powers upon the persons possessing them. Such persons could win friendship, succeed in business, tell whether they were being married for love or for money, and were safe from many diseases. Naturally enough, magnets were looked upon as valuable possessions. Yet no practical use was made of them until about the middle of the twelfth century, when a wise sailor placed a magnetized needle upon a float, to learn which way was north. Magnets are now used for many purposes, and it was the study of them and their action that led to the invention of the dynamo, the telegraph, the telephone, and many other modern conveniences.

Sailors long ago noticed that when there was constant lightning, the needle of a compass danced about in all sorts of ways. The same dancing about of the needle was observed when a magnet was brought near a compass. These and other facts, like knives being made into magnets when a house was struck by lightning, aroused curiosity among scientists, to find out what connection there was between magnetism and electricity.

In the winter of 1820, it occurred to Professor Oersted, of Copenhagen, to try a new way to find the answer. On a table before him lay a compass and beside it was one of Volta's batteries. He connected the wires to complete the circuit of the battery, and brought one wire close to the side of the compass parallel to the needle. The needle swung around, just as if he had a magnet in his hand. When the current was sent through the wire toward the north, the needle moved to the left. When the current was sent through the wire toward the south, the needle swung to the right. Oersted saw he had made a discovery. Passing an electric current through a wire makes a magnet of the wire. "Magnetism," he said, "is but electricity in motion." Oersted's discovery was of importance, for it led to the invention of the electromagnet, one of the most useful electrical inventions.

The first electromagnet was made by Sturgeon, an Englishman. He took a round bar of very soft iron and bent it in the shape of a horseshoe. Around this he wrapped a wire, and through the wire he passed an electric current. He varnished the core, as the iron bar is called, to keep the electricity from flowing off or away from it.

Sturgeon was surprised at the way the electromagnet worked. It was very much stronger than a natural or permanent magnet of the same size. But the most surprising element was that the instant the current was turned on, the iron core became a magnet, and when the current was turned off the core practically ceased to be a magnet. It might be thought that this peculiar action of the electromagnet would make it a useless plaything, but it is this very action which makes it so useful. If a needle or other object is picked up with a permanent magnet, the only way to get it off the magnet is to scrape or pull it off; but to get it off an electromagnet, it is necessary only to break the electric current. The electromagnet is thus under our control. To put it to work, we turn on the electric current; to make it stop working, we turn off the current. You do this every time you push the button of an electric doorbell.

We can control also the power of the electromagnet, that is, the size of the load it will lift. The man who taught us how to do this was Joseph Henry, an American. Instead of varnishing the iron core as Sturgeon had done, to keep the electricity from flowing off, or to insulate it, Henry insulated the copper wire by covering it with a wrapping of silk. Instead of putting a single turn of wire round the iron core, he put many turns. On his first electromagnet he put thirty-five feet of wire, making about four hundred turns. These additional turns increased the strength of the magnet very much.

Henry found that the magnet was stronger when wound with a number of separate coils of wire, the ends of each coil being connected with the battery. With a small battery, one of Henry's electromagnets lifted eighty-five pounds, and in 1831 he exhibited a magnet which lifted thirty-six hundred pounds. Thus by using a small or large battery, small or large iron cores, a few or many coils of wire, electromagnets of different strength can be made.

Henry was also the first to make the electromagnet do work at a distance, and to show us how it could be made useful. In telling of this he says: "I arranged around one of the upper rooms in the Albany Academy a wire more than a mile in length, through which I was enabled to make signals by sounding a bell." This first electric bell was made up of a permanent magnet about ten inches long, supported on a pivot, and placed with one end between the two poles of an electric magnet. When the current was passed through the electromagnet, this caused the bar magnet to swing and strike the bell.

Small electromagnets by the millions are now in use. In connection with the electric battery, they ring our doorbells, sound alarms, move signals, and the like.

Enormous lifting magnets are now employed to handle iron and steel. Some of these will lift as much as a hundred thousand pounds. Electromagnets are thus of themselves doing all kinds of work for us, and in addition they are, as we shall see, an essential part of the telegraph, the telephone, and the dynamo.