MAGNETISM. The name `magnet' is given any body which possesses the power of attracting pieces of iron. There are certain natural ores which have this power, but all magnets actually in use are artificial. The origin of the use of a magnetic compass as a means of obtaining approximately the direction of the geographical North Pole of the earth is unknown. hut the first scientific work on the laws of magnets was done by William nilbcrt (1540-1603), who published his researches and ideas in his great book Dr Magnetr. Since his time the most important work has been done in connection with electric currents. (See ELECTRICITY.) Several attempts have been made to explain magnetism, that is, to explain the magnetic action of a molecule of a magnetic substance by some electrical theory. Ampere advanced the idea that in each such molecule there is an electric current flowing in a fixed path. This idea is not, however, in accord with modern conceptions of the nature of a molecule. Weber tried to account for diamag netism by the idea of induced electric currents in the molecules. A more modern theory is one proposed tentatively by J. J. Thomson, in which molecular magnetism is regarded as due to rota tion of the molecule with its Faraday tubes connecting the atoms.
In addition to attracting iron, magnets attract pieces of other kinds of matter, e.g. nickel. cobalt, manganese, chromium, and a few others, while they repel small pieces of bismuth, anti mony, silver, and a few other substances. It was shown by Faraday that the question of repulsion or attraction depended entirely on the relative magnetic properties of the substance which is being acted on by the magnet and the material medimn in which it is immersed—this last is, of course, generally air. Those bodies which when surrounded by air are attracted toward a magnet are called 'magnetic' or `para magnetic' substances; while those which under similar circumstances are repelled are called 'diamagnetic.' (See Faraday showed, too, that there is in neither ease attrac tion or repulsion if the magnetic field is `uni form:' that is, if there is the same magnetic force felt at each point of the surrounding air.
There are two methods in general use for making artificial magnets: one is to take a bar or a needle of a magnetic substance, e.g. iron or steel, wrap an insulated wire around it like thread on a spool, and pass a strong electric current through the wire: the other is to take as before a bar or a needle of a magnetic sub stance and place it near a magnet. (A modifica tion of the latter method will he described later.) The intensity of the magnetization is increased in both cases by hammering the bar while the magnetizing action is going on. It is found that a piece of steel magnetized in this way remains so for a long time; while a piece of iron loses its magnetization.
If a number of such bar magnets or magnetic needles are made, the following facts may be observed: If a magnet in the form of a bar or needle is suspended so as to be free to rotate around a vertical pivot, as in the mariners' compass (q.v.), it will turn and place itself in a generally north and south direction. The end of the magnet which points toward the north is called the 'north pole' of the magnet: and the other, the 'south pole.' This proves that the earth itself has magnetic actions; and the plane which includes the centre of the earth and the direction of the magnetic needle at any point on the earth's surface is called the `magnetic meri dian' at that point. It should not he thought . that the `poles' of a bar magnet are definite • points. Magnetic forces may be felt over the whole magnet, but of course more intensely near the ends. In any definite case the resultant of all these forces on any one outside body will pass through a certain point in the magnet; but this point will change with the position of the outside body.
If this magnet is floated on water, it is found that there is no translation, simply a rotation around a vertical axis. The magnetic field due to the earth must he uniform for any limited region on the earth; and this experiment proves that in a uniform field the forces acting on the two ends of a bar magnet are equal and opposite. This is sometimes expressed by saying that the north and south poles of a bar magnet have equal 'strengths.' If a bar magnet is broken in two, both parts are magnets with poles at the ends. This leads to the idea that magnetism is a property of the molecules of the magnetic body. In fact, when a bar of iron is magnetized, its volume changes, its elasticity changes. etc.: if it is magnetized and demagnetized rapidly by using au alternat ing current in the magnetizing helix, its tem perature rises. (If the temperature of a magnet is raised to red heat, it loses its magnetization.) Further, if a magnet is jarred or twisted. or if its temperature is raised, its magnetization is altered. In short, any physical action which affects the molecules of a magnet alters its magnetization; and any change in the magnetiza tion alters the molecular properties. This estab lishes the fact that magnetism itself is a molecular property; and that if a molecule of a magnet could be obtained, it would have a Borth and a south pole like those of a liar magnet. It is impossible, therefore, to obtain it north pole apart from its equivalent south pole. There are no such forms of matter as magnetic conductors which enable the poles to be sepa rated. This fact is illustrated by the phenome non of induction described below.