MAGNETISM (pid to be derived from the city Maa-nesia, where the loadstone was first discovered), is the power which the magnet has to attract iron. Under DIAMAG-. NETISM it is stated that every substance is more or less affected by the magnet, but as iron is par excellence magnetic, the term is chiefly used with reference to it. Magnets are of two kinds, natural and artificial. Natural magnets consist of the ore of iron called., magnetic, familiarly known as loadstone. Artificial magnets are, for the most part, straight or bent bars of tenipered steel, which have been magnetized by the action of other magnets, or of the galvanic current.
Polarity of the Magnet. —The power of the magnet to attract iron is by no meana -equal throughout its length. If a small iron ball be suspended by a thread, and a mg- tot (fig. 1) be passed along in front of it frem one end to the other, it is powerfully attracted at the ends, but npt at all in the middle, the magnetic force increasing with the distance from the middle of the bar. The ends of the magnet where the attractive power is greatest are called its poles. By causing a magnetic needle moving horizontally to vibrate in front of the different parts of a magnet placed vertically, and counting the number o'f vibrations, the rate of increase of the magnetic intensity may be exactly found. FigA gives a graphic view of this increase. N S is the magnet; the lines n N, a a, et*, represent the magnetic intensities at the points N, a, etc., of the mag net; and the curve of magnetic intensity, N a DI a' n', is the line formed by the extremi ties of all the upright lines. It will be seen from the figure that the force of both halves, taking M as the dividing-point, is disposed in exactly the same way, that for some distance on either side of the middle or neutral point there is an absence of force, i and that its intensity increases with great rapidity towards the ends. The centers of , gravity of the areas DI N n and M S n' are the poles of the magnet, which must there— fore be situated near but not at the extremities.
A magnet has, then, two poles or centers of magnetic force, each having an equal power of attracting iron. This is the only property, however, which they possess in
common, for when the poles of one magnet are made to act on those of another, a strik ing dissimilarity is brought to light. To show this, let us sus pend a magnet, N S, fig. 3, by a band of paper, DI, hanging from a cocoon thread (a thread without torsion). When the magnet is left to itself, it takes up a fixed position, one end keeping north, and the other south. The north pole cannot be made to stand as a south pole, and vice roma; for when the magnet is disturbed, both poles return to their original posi tions. Here, then, is a striking dissimilarity in the poles, by means of which we are enabled to distinguish them as north pole and south pole. When thus suspended', let us now try the effect of another magnet upon it, and we shall find that the pole of the suspended magnet that is attracted by one of the poles of the second magnet is repelled by the other, and Dice versa; and where the one pole attracts, the other repels. If, now, the second magnet be hung like the first, it will be found that the pole which attracted the north pole of the first mat,anet is a south pole, and that the pole which repelled it is a north pole. We thus learn that each magnet has two poles, the one a north, the other a south pole, alike in their power of attracting soft iron, but differing in their action on the poles of anoth,er mag, net, likepoles repelling, and unlik,e poles attracting, each, other.
It might be thought that, by dividing a magnet at its center, the two poles could be insulated, the one half containing all the north polar magnetism, and the other the south. When this is done, however, both halves become separate magnets, with two poles in each—the original north and south poles standing in the same relation to the other two poles called into existence by the separation. We can therefore never have one kind of magnetism, without having it associated in the same magnet with the same amount of the opposite magnetism. lt is this double manifestation of force which constitutes the polar- ' ity of the magnet.