MAGNETISM 225 Fi eri- tS in fig. 13, and finally as in fig. 14, and in each position reversing it end for end will not alter the effect. All this is at once explained by the above hypothesis. A variation of the last experiment may be made thus. Place a magnet verti- n t cally, in the neigh- ^~~* f n bourhood of a mag netic needle ; by mov ing it up and down a position will be found in which the action of the magnet on the needle is wholly verti- / cal, so that the needle 5 1 ^ N is not deflected from the magnetic meridian. Now take a small piece of soft iron and move it along a line of force passing near the needle, / proceeding from the / north to the south pole of the vertical / magnet. It will then / be found, in accord- I ance with our hypo thesis, that the north pole of the needle is first repelled, and finally attracted by the soft iron. If we hang two short pieces of iron wire alongside of each other by parallel threads, they will be found to repel one another, and to hang separated by a considerable interval when a magnet is brought under them (see figs. 15 and 16). This experi ment is due to Gilbert, who rightly explains it by saying that the two ends S nearer the magnetic s 1 s pole S become like poles of opposite kind to S, while the two farther ends are like poles of the same kind as S. The experiment may be varied by placing some little distance below the pole of a magnet S a piece of mica or thin card- Fig. 15. Fig. 16. board M, and placing below that a short piece of soft iron wire; it will remain adhering to the mica, and so long as it is alone will hang more or less nearly vertical, but when another is placed alongside of it the two will diverge as in fig. 17. One of the most interesting examples of magnetic induc- I s ron tion is furnished by the action of a magnet on iron filings. n
o M If we plunge a magnet into a quantity of iron filings and then remove it, we find it thickly fringed around the poles, where the filings adhere to the magnet and to one another so as to form short bushy filaments ; the thick- ness of the fringe diminishes very rapidly towards the middle of the magnet, where very few adhere at all. These filaments are composed of magnetized particles of iron adhering by their unlike poles. If we place a small bar magnet under a piece of mode rately rough drawing paper, strewn as uniformly as possible with fine iron filings, and then tap the paper very gently so as to relieve the friction, and allow each filing to follow the magnetic action, then the filings will be seen to arrange themselves in a series of lines, passing, roughly speaking, from pole to pole, asinfig. 4(p. 222). The explanation of this phenomenon is simply that each filing becomes magnetized by induction, and, if it were quite free to move about its centre, it would not be in equilibrium until it set its longest dimension along the line of force through its centre. The roughness of the paper effectually prevents translation, but does not hinder rotation, especially when the friction is relieved by tapping ; hence every filing does actually set as if it were a little magnetic needle, subject of course to some slight disturbance from the neighbouring filings. The whole therefore assumes a grained structure, and the graining runs in the direction of the lines of force. We have thus an extremely convenient way of representing these lines to the eye, which lends itself in a variety of ways to the illustration of magnetic phenomena. In fig. 5 are shown the lines formed in the field near two like magnetic poles. These magnetic figures may be fixed in a great variety of ways, and projected on a screen so as to be visible to a large audience, but it is scarcely necessary to dwell here upon details of this kind. These magnetic curves seem to have fixed the attention of natural philosophers at a very early period. They were originally called the magnetic currents, from an idea that they represented the stream lines of magnetic matter, which explained the magnetic action according to the theory then in vogue. La Hire mentions them, Mem. de I Acad., 1717. Bazin gives an elaborate account of them in his Description des Courans Magnetiques dessines tfapres Nature, Stras- burg, 1753. Musschenbroek seems to have been the first to give the correct explanation depending on magnetic induction, Diss. de Magnete, 1729. If the filings be laid very thickly on the paper, and one Lucre- pole of the magnet be brought under them at a short tius s ex- distance off, they will arrange themselves in a pattern, P er > ment and at the same time bristle up so as to stand more or less erect, according as they are nearer or farther from the magnet. They have thus the appearance of being repelled from the magnet. It was, in all proba bility, this phenomenon that was observed by Lucretius when he says (vi. 1042): " Exultare etiam Samothracia ferrca vidi, Ac ramenta simul ferri furere intus ahenis In scaphiis, lapis hie Magues cum subditus esset. His conclusion, therefore, that iron sometimes flies and sometimes follows the magnet, was scarcely justified by his experimental facts, and it is a mistake to suppose, as some have done, that he was aware of the polarity of permanent magnets. If we tap the card in the last experiment a curious Magnetic result may sometimes be observed. 1 The lines of paradox, filings will be seen to recede from the point of the card immediately over the pole of the magnet. If, however, the magnet be held over, instead of under, the card, tapping will cause the filings to approach the point under the pole of the magnet. The most probable explanation 2 of this is to be found in the fact that the erected filings stand in the Tentamcn Theorise Electricitatis et Magnctismi, 1759 ; Cavallo, Treatise on Magnetism, 1787. 2 Koget, Library of Useful Knowledge, 1832.
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