94. Diamagnetism. - A number of bodies, notably bismuth, antimony, phosphorus, and copper, are apparently repelled from the poles of a magnet. Such bodies are called diamagnetic bodies; a fuller account of them will be found in Lesson XXIX.

95. The Earth a Magnet. The greatest of Gilbert's discoveries was that of the inherent magnetism of the earth. The earth is itself a great magnet, whose "poles" coincide nearly, but not quite, with the geographical north and south poles, and therefore it causes a freelysuspended magnet to turn into a north-and-south position. Gilbert had some lodestones cut to the shape of spheres to serve as models of the globe of the earth. Such a globular magnet he called a terrella. He found that small magnets turned toward the poles of the terrella, and dip, as compass-needles do, toward the earth.

The subject of Terrestrial Magnetism is treated of in Lesson XII. It is evident from the first law of magnetism that the magnetic condition of the northern regions of the earth must be the opposite to that of the northseeking pole of a magnetized needle. Hence arises the difficulty alluded to on page 92.

Magnetism may be

96. Induction of Magnetism. communicated to a piece of iron without actual contact

Fig. 58.

with a magnet. If a short, thin unmagnetized bar of iron be placed near some iron filings, and a magnet be brought near to the bar, the presence of the magnet will induce magnetism in the iron bar, and it will now attract the iron filings (Fig. 58). This inductive action is very similar to that observed in Lesson III. to take place when a non-electrified body was brought under the influence of

an electrified one. The analogy, indeed, goes further than this, for it is found that the iron bar thus magnetized by induction will have two poles; the pole nearest to the pole of the inducing magnet being of the opposite kind, while the pole at the farther end of the bar is of the same kind as the inducing pole. Those bodies in which a magnetizing force produces a high degree of magnetization are said to possess a high permeability. It will be shown presently that magnetic induction takes place along certain directions called lines of magnetic induction, or lines of magnetic force, which may pass either through iron and other magnetic media, or through air, vacuum, glass, or other non-magnetic media: and, since induction goes on most freely in bodies of high magnetic permeability, the magnetic lines are sometimes (though not too accurately) said to "pass by preference through. magnetic matter," or, that "magnetic matter conducts the lines of force."

97. Attraction across Bodies.. - If a sheet of glass, or wood, or paper, be interposed between a magnet and the piece of iron or steel it is attracting, it will still attract it as if nothing were interposed. A magnet sealed up in a glass tube still acts as a magnet. Lucretius found a magnet put into a brass vase attracted iron filings through the brass. Gilbert surrounded a magnet by a ring of flames, and found it still to be subject to magnetic attraction from without. Across water, vacuum, and all known substances, the magnetic forces will act; with the single apparent exception, however, that magnetic force will not act across a screen of iron or other magnetic material, if sufficiently thick. If a small magnet is suspended inside a hollow ball made of iron, no outside magnet will affect it. The reason being that the magnetic lines of force are conducted off laterally through the iron instead of penetrating through it. A hollow shell of iron will therefore act as a magnetic cage, and shield the space inside it from magnetic influences.

Fig. 59 illustrates the way in which a cylinder of soft iron shields the space interior to it from the influence of an external magnet. A compass needle placed at P inside the cylinder is not affected by the presence of the magnet outside, for its lines of magnetic force are drawn off laterally. Similarly a magnet inside is shielded from affecting outside space.

Although magnetic induction takes place at a distance across an intervening layer of air, glass, or vacuum, there is no doubt that the intervening medium is directly concerned in the transmission of the magnetic force, though the true medium is probably the "ether" of space sur

N

Fig. 59.

rounding the molecules of matter, not the molecules themselves.

We now can see why a magnet should attract a notpreviously-magnetized piece of iron; it first magnetizes it by induction and then attracts it: for the nearest end will have the opposite kind of magnetism induced in it, and will be attracted with a force exceeding that with which the more distant end is repelled. But induction precedes attraction.

98. Retention of Magnetization. Not all magnetic substances can become magnets permanently. Lodestone, steel, and nickel retain permanently the great part of the magnetism imparted to them. Cast iron and many impure qualities of wrought iron also retain

H

magnetism imperfectly. The softer and purer a specimen of iron is, the more lightly is its residual magnetism retained. The following experiment illustrates the matter:- Let a few pieces of iron rod, or a few soft iron nails be taken. If one of these (see Fig. 60) be placed in contact with the pole of a permanent steel magnet, it is attracted to it, and becomes itself a temporary magnet. Another bit of iron may then be hung to it, and another, until a chain of four or five pieces is built up. But if the steel magnet be removed from the top of the chain, all the rest drop off, and are found to be no longer magnetic. A similar chain of steel needles may be formed, but they will retain permanently most of their magnetism.

Fig. 60.

It will be found, however, that a steel needle is more difficult to magnetize than an iron needle of the same dimensions. It is harder to get the magnetism into steel than into iron, and it is harder to get the magnetism out of steel than out of iron; for the steel retains the magnetism once put into it. This power of resisting magnetization, or demagnetization, is sometimes called coercive force; a much better term, due to Lamont, is retentivity. The retentivity of hard-tempered steel is great; that of soft wrought iron is very small. The harder the steel, the greater its retentivity. Form affects retentivity. Elongated forms and those shaped as closed or nearly closed circuits retain their magnetism better than short rods, balls, or cubes.

99. Theories of Magnetism. The student will not have failed to observe the striking analogies between the phenomena of attraction, repulsion, induction, etc., of magnetism and those of electricity. Yet the two sets

of phenomena are quite distinct. A positively electrified body does not attract either the North-pointing or the South-pointing pole of the magnet as such; in fact, it attracts either pole quite irrespective of its magnetism, just as it will attract any other body. There does exist, indeed, a direct relation between magnets and currents of electricity, as will be later explained. There is none known, however, between magnets and stationary charges of electricity.

In many treatises it is the fashion to speak of a magnetic fluid or fluids; it is, however, absolutely certain that magnetism is not a fluid, whatever else it may be. The term is a relic of bygone times. A magnet when rubbed upon a piece of steel magnetizes it without giving up or losing any of its own magnetism. A fluid cannot possibly propagate itself indefinitely without loss. The arguments to be derived from the behaviour of a magnet on breaking, and from other experiments narrated in Lesson X., are even stronger. No theory of magnetism will therefore be propounded until these facts have been placed before the student.

LESSON IX. Methods of making Magnets

100. Magnetization by Single Touch. It has been so far assumed that bars or needles of steel were to be magnetized by simply touching them, or stroking them from end to end with the pole of a permanent magnet of lodestone or steel. In this case the last touched point of the bar will be a pole of opposite kind to that used to touch it; and a more certain effect is produced if one pole of the magnet be rubbed on one end of the steel needle, and the other pole upon the other end. There are, however, better ways of magnetizing a bar or needle.

101. Magnetization by Divided Touch. In this method the bar to be magnetized is laid down hori