The fact of the freely suspended magnet taking up a fixed position has led to the theory that the earth itself is a huge magnet, having its north and south magnetic poles in the neighborhood of the poles of the axis of rotation, and that the magnetic needle or suspended magnet turns to them as it does to those of a neighboring magnet. All the manifestations of terrestrial magnetism give decided confirmation of this theory. It is on this view that the French call the north pole of the magnet the south pole (pole .austral), and the south the north pole (pole boreal); for if the earth be taken as the stand ard, its north magnetic pole must attract the south pole of other magnets, and vice versa. In England and Germany the north pole of a magnet is the one which, when freely suspended, points to the north, and no reference is made to its relation to the magnetism of the earth.
Form of Magnek—Artificial magnets are either bar magnets or horse-shoe magnets. 'When powerful magnets are to be made, several thin bars are placed side by side, witli rtheir poles lying in the same way. They end in a piece of iron, to vvhich they art bound by a brass screw or frame. Three or four of these may be put up into the bundle, 1:and these again into bundles of three and four. Such a collection of magnets is called a 'magnetic magazine or battery. A magnet of this kind is more powerful than a solid one of the same weight and size, because thin bars can be more strongly and regu larly magnetized than thick ones. Fig. 4 is a horse-shoe magnet magazine. The central lamina protrudes slightly beyond the other, and it is to it that the armature is attached, the whole action of the magnet being concentrated on the projection. A good form of magnet is a parallelopiped of magnetic iron ore, with pieces of soft iron, bound to its poles by a brass frame encircling the whole. The lower ends of the soft iron bars act ms the poles, and support the armature. The magnetic needle is a small magnet nicely balanced on a fine point. See Comrass.
Magnetic induction. —When a short bar of soft iron is Suspended from one end of a magnet it becomes for the time powerfully magnetic. It assumes a north and south pole, like a regular ma,,frnet, as may be seen by using a small magnetic needle; and if its lower end be dipped into iron filings, it attracts them as a magnet would do. Virhen it is taken away from the magnet the filings fall off, and all trace of magnetism pears. It need not be in actual contact to show magnetic properties; when it is simply brought near, the same thing is seen, though to a less extent. If the inducing inagnet
be strong enough, the induced magnet, when in contact, can induce a bar like itself, placed at its extremity, to became a magnet; and this second induced magnet may .mit the magnetism to a third, and so on, the action being, however, weak* each time. If a steel bar be used for this expenment, a singular difference is observed in its action; it is only after sonie time that it begins to exhibit magnetic properties, and, when exhibited, they are feebler than in the soft iron bar. When the steel bar is retnoved, it does not part instantly with its magnetism, as the soft iron bar, but retains it manently. Steel, therefore, has a force which, in the first instance, resists the assumption of magnetism; and, when assumed, resists its withdrawal. This is called the coercitive force. The harder the temper of the steel, the more is the coercitive force developed in it. It is this force also, in the loadstone, which enables it to retain its magnet ism.
Magnetization.—By single touch (Fr. simple touche, Ger. einfacher strich): The steel bar to be magnetized is laid on a table, and the pole of a powerful magnet is rubbed a few times along its length, always in the same direction. If the magnetizing pole be north, the end of the bar it first touches each time becomes also north, and the one where it is lifted south. The same thing may be done by putting, say, the north magnetizing pole first ou the middle of the bar, then giving it a few passes from the middle to the end, returning always in an arch from the end to the middle. After doing the same to the other half with the south pole, the magnetization is complete. The first end rubbed becomes the south, and the other the north pole of the new mac, net. By divided touch (Fr. touche separee, Ger. getrennter stride): The bar to be magnet ized is placed on a piece of wood with its ends abutting on the extremities of two power ful magnets. Two rubbing magnets are placed with their poles together on the middle, inclined at an angle rather less than 30° with it. They are then simultaneously moved Away from each other to the ends, and brought back in an arch again to the middle. After this is repeated a few times; the bar is fully magnetized. This method communi •cates a very regular magnetistn, and is employed for magnetic needles, or where Accuracy is needed. The magnetization by double touch is of less practical importance, -and need not here be described. It communicates a powerful but sometimes irreg-ular ; magnetism, giving rise to consecutive poles—that is, to more poles than two in the -magnet.