#### What is magnetism? How can a current be produced by magnetism? How do we use mechanical energy to produce electric current?

Cells and batteries are often convenient sources if electrical energy. For some purposes, we use them a great deal. But if we had to rely entirely on current produced by chemical action, we could not afford to operate most of our common electrical appliances. The metals and chemicals used up in the cells would cost too much. Some other way of producing current had to be found before we could have electrical energy in the large amounts that we need today. About 1830, two scientists working separately discovered how to produce electric current by magnetism. One of them was an American named Joseph Henry and the other was an Englishman named Michael Faraday. From their discovery, the electric generator was developed.

As you have learned, a generator is a machine that changes mechanical energy into electrical energy. Generators can produce large amounts of electrical energy much more cheaply than cells can. For example, it costs anywhere from seven to thirty times as much to get electrical energy from a dry cell as from a generator. When a large current is needed for very long, generators are nearly always used to supply it. More than 99 per cent of all the electrical energy now used in our country comes from generators. To understand how they supply current, you must first have some facts about magnetism clearly in mind.

Magnetism is a form of energy. Because of this energy, a magnet exerts a force. It attracts magnetic materials, such as iron and steel. The places where the force of a magnet is strongest are called its poles. Every magnet has two poles, usually near the ends. One is called the north pole, or N pole. The other is called the south pole, or S pole. The space around a magnet where the force acts is known as its field of force, or magnetic field. Here iron filings arrange themselves in a pattern made up of a series of lines. We call them lines of force, because the force of a magnet seems to act along these lines. They pass between the poles through the field of force. So the lines of force show us how the magnetic field is arranged.