22G former case as shown in fig. 18, and in the latter as shown in fig. 19 ; that is, in both cases, owing to the action of gravity, they are more acutely inclined to the card than !J N Fig. 18. the lines of force (represented by dotted lines in the figure). Consequently, when the filing springs up into the air, and is thus free to follow the magnetic couple, it turns more Electro magnets. Induced and per manent magnet ism in the same body. into the direction of the line of force ; the effect of this is to carry its lower end each time a little farther from the axis of the magnet in the one case, and a little nearer to it in the other. By far the most important case of magnetic induction is the electromagnet. Whenever an electric current flows in a closed circuit, the surrounding space becomes a field of magnetic force, and any piece of iron in it will be in ductively magnetized. Such an arrangement of an electric circuit and iron is called an electromagnet. The variety of form and of application of such instruments in modern science is endless. A few of the more important modifica tions will be considered below. Go-existence of Induced and Permanent Magnetism. The fact that a body is already a permanent magnet does not prevent its being susceptible to magnetic induction. If we take any piece of iron at random, the chances are that one end or other of it will repel the north pole of a magnetic needle, in other words, it will be to some extent per manently magnetic ; but if we bring it slowly nearer and nearer to the pole of the needle, provided its magnetism be not too strong, it will by and by attract the pole which it at first repelled. Again, if we take two steel magnets, which may be as powerful as we please, provided at all events that they are unequally powerful, and bring two like poles together, these poles will at first repel each other in accordance with the fundamental law of permanent magnets; but, when the distance is less than a certain amount, the repulsion passes into an attraction, and when the poles are in contact this attraction may be very considerable. These phenomena are at once explained by the law of induction. The induced or temporary magnetism is superposed on the permanent magnetism, and, when the poles are near enough, the opposite magnetism induced by the pole attracts it more than the permanent like magnetism repels it ; and this happens even with steel, whose susceptibility for magnetic induction is considerably less than that of iron. This phenomenon was observed pretty early in the history of magnetism, but was not fully explained until the idea of magnetic induction was fully developed. Michell, in his Treatise of Artificial Magnets, 1 gives a tolerably clear account of it. Musschenbroek mentions it, 2 along with the fact that a magnet attracts iron more than it does another magnet, but offers no explanation of either fact. The latter result, so far as it is true, can of course be explained by the smaller susceptibility of steel, particularly of hard steel, to magnetic induction, which is the main factor in attraction at small distances. Poggendorff 3 arid others have experimented on the subject in later times. The reader should notice the close analogy between the^e phenomena and the repulsion and attraction at different distances between two similarly electrified conductors. See article ELECTRICITY, vol. viii. p. 33. Induction of Permanent Magnetism. The case above supposed, in which the induced magnetism is wholly temporary, although it can be easily realized with small magnetizing forces, is not the general one, but in fact the exception. Usually a certain proportion of the magnetism remains after the inducing force is removed. This happens Co -rciv even with the softest iron, when the inducing force is very for-.-e. great. Just as bodies differ very much in their suscepti bility for induced magnetism, so they differ greatly in their power of retaining this magnetism when the inducing force ceases, or, as the phrase is, in "coercive force." Thus, while the inductive susceptibility of steel is less than that of iron, it retains much more of the magnetism imparted to it, and is therefore said to have much greater coercive force ; and the coercive force is greater the harder the steel is tempered. It is obvious, therefore, that the principle of " induction," along with the idea of "retaining power" or "coercive force," furnishes us with the key to the explanation of the com munication of permanent magnetism, whether by means of natural magnets or of artificial magnets, or of the electric current. In particular, we see at once the reason why the end of a needle which has been touched by the north pole of another magnet becomes a south pole, and vice verso, a fact which greatly puzzled the earlier magnetic experi menters, and indeed all who were inclined to think that, in the process of magnetization, something was communicated from the one magnet to the other. MATHEMATICAL THEORY OF THE ACTION OF PERMA NENTLY MAGNETIZED BODIES. In this section we shall suppose the bodies considered to be rigidly magnetized ; i.e., we shall suppose that magnetic action exerted on any body produces no change in its magnetization. It is further to be observed that we are merely establishing a compendious representation of ob served facts, and foreclosing nothing as to their physical theory or ultimate cause. Our method is therefore to some extent tentative, and its success is to be judged by the agreement of the results with experiment. There are two main facts to be borne in mind : (1) that a magnet is polarized, and (2) that the properties of its smallest parts are similar to those of the whole. Adopting the mathematical fiction of action at a distance, we may represent the action of such a body by a proper distribution of imaginary positive and negative attracting matter through- out its mass. This imaginary matter, following Sir W. Thomson, we shall call "magnetism," as we thus avoid suggesting other properties of matter than attraction, of which in the present case experience has given no evidence. We assume that magnetism of any sign repels magnetism 1 Cambridge, 1750. 2 Phnosoihia Naturalis, 953, 954, 1762. 3 Pogg. Ann., xlv. p. 375, 1838. Positive
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