Fig. 1
There are three primary kinds of magnets: the lodestone or natural magnets, the artificial or permanent steel magnet, and the electric magnet. At present the lodestone is little used. The permanent steel magnet is used for compass needles, as the familiar horseshoe magnet, and in certain types of electric machinery. The electric magnet forms a part of nearly every kind of electrical machinery and is by far the most useful form of the magnet. The modern high-duty lifting magnet is a form of the electric magnet.
The properties of the lodestone and the permanent magnet have been known for thousands of years, while the electric magnet is a comparatively recent discovery.
All magnets, whether natural, permanent or electric, possess the same magnetic properties. Every magnet has two poles commonly called a north pole and a south pole. It has also been found that when a magnet is broken in two each piece becomes a magnet in itself with its own north and south poles.
For practical purposes it has been found convenient to assume that magnetism consists of a series of “lines of force” running through the magnet from one end to the other and back again through the air. Each one of these lines is assumed to have a certain strength, and the power of any magnet is determined by the number of lines of force flowing through it. These lines are clearly shown in [Fig. 1], which was made by sprinkling iron filings on a sheet of paper over a bar magnet, and tapping the paper slightly so that the filings could arrange themselves along the magnetic lines of force.
Since Oersted’s first electric magnet in 1820, electric magnets have been made in a variety of forms and for many different purposes. The simplest form of electric magnet is shown in [Fig. 2]. It consists of an iron bar with an insulated electric wire wound around it carrying an electric current.
Fig. 2