Magnet, Fig. 20, is, however, the one with which we are the most familiar. They are always painted red, but the red paint has nothing to do with the magnetism.

Fig. 21.

Fig. 22.

The little end-piece is called the keeper, or armature; it should always be kept in place when the magnet is not in use. The magnet itself is made of steel, while the armature is made of soft iron. Steel retains magnetism for a long time, while soft iron loses it almost instantly. The ends of the magnet are called its poles, and nearly all the strength of the magnet seems to reside at the poles, the curved part having no attraction for outside bodies. One of the poles of the magnet is marked with a line, or with the letter N. This is called the north pole of the magnet, the other being its south pole.

27. Bar Magnets are straight magnets. Fig. 21 shows a round bar magnet. The screw in the end is for use in the telephone, described later.

28. Compound Magnets. When several thin steel magnets are riveted together, a compound magnet is formed. These can be made with considerable strength. Fig. 22 shows a compound horseshoe magnet. Fig. 23 shows a form of compound bar magnet used in telephones. The use of the coil of wire will be explained later. A thick piece of steel can not be magnetized through and through. In the compound magnet we have the effect of a thick magnet practically magnetized through and through.

Fig. 23.

29. Magnetic and Diamagnetic Bodies. Iron, and substances containing iron, are the ones most readily attracted by a magnet. Iron is said to be magnetic. Some substances, like nickel, for example, are visibly attracted by very strong magnets only. Strange as it may seem, some substances are actually repelled by strong magnets; these are called diamagnetic bodies. Brass, copper, zinc, etc., are not visibly affected by a magnet. Magnetism will act through paper, glass, copper, lead, etc.

Fig. 24.

30. Making Magnets. One of the strangest properties that a magnet has is its power to give magnetism to another piece of steel. If a sewing-needle be properly rubbed upon one of the poles of a magnet, it will become strongly magnetized and will retain its magnetism for years. Strong permanent magnets are made with the aid of electromagnets. Any number of little magnets may be made from a horseshoe magnet without injuring it.

Fig. 25.

31. Magnetic Needles and Compasses. If a bar magnet be suspended by a string, or floated upon a cork, which can easily be done with the magnet made from a sewing-needle, Fig. 24, it will swing around until its poles point north and south. Such an arrangement is called a magnetic needle. In the regular compass, a magnetic needle is supported upon a pivot. Compasses have been used for many centuries by mariners and others. Fig. 25 shows an ordinary pocket compass, and Fig. 26 a form of mariner's compass, in which the small bar magnets are fastened to a card which floats, the whole being so mounted that it keeps a horizontal position, even though the vessel rocks.

Fig. 26.

32. Action of Magnets Upon Each Other. By making two small sewing-needle magnets, you can easily study the laws of attraction and repulsion. By bringing the two north poles, or the two south poles, near each other, a repulsion will be noticed. Unlike poles attract each other. The attraction between a magnet and iron is mutual; that is, each attracts the other. Either pole of a magnet attracts soft iron.

In magnetizing a needle, either end may be made a north pole at will; in fact, the poles of a weak magnet can easily be reversed by properly rubbing it upon a stronger magnet.

33. Theory of Magnetism. Each little particle of a piece of steel or iron is supposed to be a magnet, even before it touches a magnet. When these little magnets are thoroughly mixed up in the steel, they pull in all sorts of directions upon each other and tend to keep the steel from attracting outside bodies. When a magnet is properly rubbed upon a bar of steel, the north poles of the little molecular magnets of the steel are all made to point in the same direction. As the north poles help each other, the whole bar can attract outside bodies.

By jarring a magnet its molecules are thoroughly shaken up; in fact, most of the magnetism can be knocked out of a weak magnet by hammering it.

34. Retentivity. The power that a piece of steel has to hold magnetism is called retentivity. Different kinds of steel have different retentivities. A sewing-needle of good steel will retain magnetism for years, and it is almost impossible to knock the magnetism out by hammering it. Soft steel has very little retentivity, because it does not contain much carbon. Soft iron, which contains less carbon than steel, holds magnetism very poorly; so it is not used for permanent magnets. A little magnetism, however, will remain in the soft iron after it is removed from a magnet. This is called residual magnetism.

Fig. 27.

35. Heat and Magnetism. Steel will completely lose its magnetism when heated to redness, and a magnet will not attract red-hot iron. The molecules of a piece of red-hot iron are in such a state of rapid vibration that they refuse to be brought into line by the magnet.

36. Induced Magnetism. A piece of soft iron may be induced to become a magnet by holding it near a magnet, absolute contact not being necessary. When the soft iron is removed, again, from the influence of the magnet, its magnetism nearly all disappears. It is said to have temporary magnetism; it had induced magnetism. If a piece of soft iron be held near the north pole of a magnet, as in Fig. 27, poles will be produced in the soft iron, the one nearest the magnet being the south pole, and the other the north pole.

Fig. 28.

37. Magnetic Field. If a bar magnet be laid upon the table, and a compass be moved about it, the compass-needle will be attracted by the magnet, and it will point in a