Theory of Magnetism. 735 then to draw the magnet steadily along, held at the same angle, to B. After repeating this touch several times on one side, the other end of the magnet, E, is brought to the centre of the bar, C, and is drawn along to A, and an equal number of passes being made as from C to B, the bar, A B, is permanently magnetized. Instead of thus using a single magnet, C E, for both ends, two similar magnets may be applied at the same time. A bar of iron or steel, by merely lying long near a strong magnet, may become magnetic by influence or induction. Now this great globe which we inhabit, is proved by its influence on the compass needle to behave exactly like a huge magnet stretching from the north to the south pole; and in like manner a large mass of iron, or a bar of steel, lying long in the N. and S. direction acquires magnetism by the inductive influence of the earth. In this way, without the possession of any magnet at all, we may easily procure one for ourselves: holding a bar of tempered steel in the north and south line, and striking it a few times with a hammer on one end, we get a magnetized steel bar, or a bar-magnet, directly produced by the power of the earth. Workmen's tools, or a steel poker, are thus almost always found to be more or less magnetic. 972. Theory of Magnetism.—In searching for an explanation of this curious unseen power of attraction,—for the mind of man is never satisfied until it perceives all the connecting links between cause and effect,—philosophers have held several different theories. The simple view of the primitive philosophers supposed the existence of two material magnetic fluids of opposite character; an accumulation of the one at the north pole, and of the other at the south pole, was all the explanation of magnetization which this required. But the phenomena of electric currents, besides many others observed in modern times, require some totally different hypothesis. There can be little doubt that magnetism is a molecular affection. For, in the first place, if a steel bar-magnet, such as a magnetized piece of watch-spring, be broken in two, each part is jas complete a magnet as the original; and however often we break it, the minutest fragment is a perfect magnet, showing that the polarity or duality of character which the original bar possessed is equally a property of its every molecule. In the second place, magnetization is accompanied with molecular disturbance. It has been found by Joule, of Manchester, that an iron bar becomes slightly elongated when magnetized, while its width is correspondingly reduced; just as if there were a re-arrangement of the mole 736 The Electro-magnet. cules among themselves, as if each turned round and set with its greatest length in the axis of the bar. In the third place, heating— and this we have seen is a molecular affection—or striking a magnet disturbs, or may even destroy, its magnetism.* The molecular theory of magnetism is supported by many other considerations. If a thin glass tube be filled with steel-filings, and a strong magnet passed along it several times, it is found that the whole tube acts as one magnetic bar, having its ends oppositely magnetized; on disturbing the arrangement of the filings by shaking it, we find that every trace of the polarity has vanished. The electric current, again, is due, as we have seen, to molecular motions or disturbances in the cells of the battery, which are transmitted along the wire; and it accords with the molecular theory of magnetism that the flow of such a current round a bar of soft iron, produces the magnetic state of the iron. This is, in fact, one of the readiest as well as the most powerful ways of magnetizing a steel bar, or of producing temporary magnetism in an iron bar. For there is this remarkable difference between iron and tempered steel-due also to the change of molecular character in the latter-that, while a bar of iron can be much more readily magnetized in this way by a coil of cotton-covered wire conveying a current round it, yet it cannot acquire a permanent magnetic state. It is magnetic only so long as the current flows. On the other hand, a bar of steel does not take up the magnetic molecular arrangement with such facility as the soft iron; but it is all the more permanent that it is slow. The cause is considered to be, that in the tempered steel the particles, or molecules, are not so readily wheeled round into the required set direction as in the soft iron; or that the soft iron molecules are in fact the more fluid of the two. 973. The Electro-magnet.-Fig. 276 represents a convenient form of a powerful electro-magnet; it consists essentially of two *The effect of heat admits of easy demonstration. Let a suspended magnet be placed near a bar of iron, about eight inches long and about three-quarters of an inch square in section, so that the magnet is drawn out of its north and south position. The bar should be laid on a brick. If the end of the bar is now made red-hot, and replaced in its position near the magnet, it will be found that the magnetism of the bar has entirely disappeared, and the magnet flies off to the north. As the bar cools, and reaches a low, black heat, the magnet is drawn from its position, and is again attracted by the cold bar. Heat appears here to replace temporarily magnetic force. coils, C c', of thick cotton-covered copper wire, which are fitted on two massive iron rods connected at the bottom by a thick flat piece of iron, I, which, being screwed in the wooden base or stand, serves to keep the magnet upright. Two movable C P masses of soft-iron, P P', serve as movable poles of the magnet, which may be adjusted in any required position. If the two poles, P and P', be placed with their flat ends meeting, and a strong current, say, that from four to six Bunsen or Grove cells (see Art. 961), the two pieces, P P', will be locked fast together, so that it will be impossible to separate them by the mere hand. But the moment the current is stopped, as by removing one of the wires from the binding screw, S, the magnet is shorn of its strength, and the iron becomes powerless as before. There is no limit to the strength of a magnet which may be thus constructed; tons may be supported in this way, and dropped as by magic through the mere touch of a child removing the current-conveying wire. If a piece of stout pasteboard be laid over the poles of such a magnet, and a heap of iron filings inverted on the pasteboard, they become like a soft viscous mass, which may be baked almost into any shape, or the filings will sprout up into shrub-like forms as if they were endowed with a sudden living power of growth.* Fig. 276. 974. Diamagnetism.-But, perhaps, the most remarkable revelations which such a powerful magnet makes are, that, although iron and its varieties are apparently the only magnetic substances, still all bodies are in a greater or less degree affected by magnetism. It has been detected by means of powerful steel magnets that nickel and cobalt were feebly attracted by the magnet, while bismuth and antimony were repelled. These, however, were only known as supposed curious exceptions to the general non-magnetic character of all other metals. Faraday, with a powerful electro-magnet, was the * In an experiment at the Royal Institution, Faraday suspended to his powerful electro-magnet, two sets of heavy fire-irons and a coal-scuttle full of coals. The watches of many of his audience sitting on the front benches were scriously damaged by reason of the inductive influence of this powerfu! magnet operating for a considerable distance. 738 Terrestrial Magnetism. first to prove that all substances fall under one or other of the two classes named by him paramagnetic and diamagnetic. Substances belonging to the former class are attracted by the magnet, and a little rod of a paramagnetic substance sets lengthways between the pointed ends of the movable poles of the magnet; it includes the metals iron, nickel, cobalt, manganese, chromium, palladium, and platinum; also oxygen gas and air. Diamagnetic bodies, on the other hand, are repelled by either pole of a magnet, and a diamagnetic bar sets as represented in fig. 276, not lengthways, but right across the direction of the poles ; of this class are bismuth, antimony, mercury, lead, tin, silver, zinc, gold, copper, water, alcohol, sulphur, resin, wax, sugar, starch, wood, ivory, leather, bread, and other organic substances. Professor Tyndall has also found that crystals, when suspended between the poles of an electro-magnet, behave in such a way as to show that there is some intimate relation between what is known as their optic axis and the line of most powerful magnetic action. The theory of magnetism which is known by the name of Ampere's theory is, that the magnetism of a steel bar is akin to that of the electric current; in fact, that minute electric currents (whose origin and source are, however, beyond speculation) circulate round the component molecules of a steel magnet; the difference between an unmagnetic and a magnetic bar being that, in the latter, the electric currents are all disposed in one direction, whereas in the former they are in all different directions, and so mutually destructive. A similar theory has been advanced to account for the magnetism of the earth; huge electric currents circulating round our globe, and caused probably by the thermo-electric action of the sun's rays, are considered quite a legitimate and sufficient explanation of the phenomenon. ន TERRESTRIAL MAGNETISM. 975. It might be supposed, at first, that a magnetic needle free to move, say, hung up by a very fine fibre, or floating on water on a cork support, would be drawn towards the north pole of the earth; but we must remember that if the point of the balanced needle be attracted by the north pole of the earth, then the eye end is also repelled by it; and the force of repulsion will differ from the force of attraction by an inappreciable Fig. 277. Variations of the Compass. 739 degree, seeing that the difference of the distances of the two poles from the north pole of the earth is less than the hundred-millionth part of the whole distance. There is a like attraction and repulsion by the south pole of the earth on the two ends of the needle; hence the resulting action is merely directive, that is to say, merely turns the needle round, so as to lie in the line of these two opposite forces. For a similar reason, two magnetic needles, A and B, of exactly equal strength, if fixed together with their two poles reversed, as in fig. 278, assume no particular direction as regards the earth; while they still exert and exhibit their separate powers on objects near to them. Such a double needle is called astatic, or without fixed direction. N Fig. 278. N 976. Variations of the Compass.—It is, however, an important and singular fact, to be noted, that the compass-needle does not point exactly north and south, that is to say, the magnetic poles do not quite SW N NE SE Fig. 279. E the west of north; in other words, to find the direction of the true north from a compass, we must reckon it 19° to the east of the direction of the needle, a point corresponding nearly to the position of the north pole-star. In the year 1657, more than two hundred years ago, there was in England no deviation of the compass, its direction being true north and south. Prior to that year, it had deviated to the east of the true north; but after that year, it slowly deviated more and more towards the west, till it attained a maximum deviation about the year of the battle of Waterloo. Since then its westerly declination has been slowly lessening, being now very nearly 19° west.* *In 1576 the variation was 11° 15′ E. The needle gradually fell back to the north, and from 1657 to 1662 it pointed due north. It then passed to the west of north, reaching 8° west in 1700, and 24° west in 1800. The |