Magnetism (magnet, "the lodestone") is a special condition of a body, readily recognised by well-known properties. Those bodies that exhibit such properties are termed magnetic, and are best exemplified in iron and steel. The earliest observations were made on natural magnets of lodestone, and from remote periods. One practically important property of a magnet, was known, that if suspended horizontally by a fibre or on a float it would turn into a definite direction and point approximately to the geographical north and south. Magnetism may be induced in a magnetic substance by simply placing it in the neighbourhood of a, magnet, the surrounding medium evidently being in a condition different from that which obtains when no magnet is near. Moreover, the presence of a highly magnetic substance in a medium in such a condition, renders it more difficult to magnetise a less magnetic substance by simply placing it in the same neighbourhood. In fact, magnetism may be treated quantitatively, and any increase in the intensity of a portion of the magnetic field as the magnetised medium is called, due simply to the introduction of magnetic matter, is accompanied by a decrease in another portion. The magnetised state is dual; there are two different conditions in the one magnet. If the substance is in the form of a bur. and the whole length of this has been treated as uniformly as possible in the production of the magnet, the two ends will exhibit opposite effects, the end that points towards the geographical north being called the north pole, and the other end the south pole. It will be found experimentally that the north pole will attract the south pole of any other magnet, but will repel its north pole. The south pole, on the other hand, will attract north and repel south. Like poles thus repel each other, and unlike poles attract each other. If a north pole of a magnet be remote from its south pole, its north will be found, when placed in the neighbourhood of another magnet, to travel away from the north or towards the south of the second. Also it is found that the attraction in the one case and repulsion in the other is proportional to the inverse square of the distance between the two poles. The strength of a magnetic pole is usually measured by determiningits force of attraction or repulsion on a known pole at a known distance, the force at a definite distance being proportional to the product, of the strengths of the two poles. A bar of steel may be magnetised by rubbing one pole of a magnet along its whole length several times, providing that the bar is only rubbed in one direction. Any reversal of the direction of rubbing neutralises part of the magnetisation. A more effective method is to start at the middle with two unlike poles of two magnets and rub the bar in opposite directions. Steel possesses the property of retaining the magnetic condition after the magnetising force is withdrawn. Careless handling may partially destroy the magnetisation; violent heating will do so entirely. The theory that magnetisation involves a rearrangement of the particles of the substance is supported by the facts that magnetisation of a bar is accompanied by change of volume, that rapid alternations of its magnetism cause a distinct humming sound, and that the retentivitg of a substance for magnetism or its reluctance to part with it varies with its composition. Wrought-iron is much more easily magnetised when placed in a magnetic field, but much more easily demagnetised when removed from it. The power of a substance for concentrating magnetism within itself is termed its permeability, and wrought-iron is by far the most powerful substance in this respect. Most substances - bismuth, for example - are less capable of retaining magnetism than the surrounding air medium. The bismuth will appear to be repelled by a magnet, and will assume a position at right angles to the direction assumed by a magnetic needle. Such substances are termed diamagnetic.
The most, powerful means of inducing magnetism in a substance is by means of an electric current flowing round a coil of wire. This is due to the fact that such an electric current affects the medium in exactly the same way as a magnet, and any bar with great permeability, placed in the neighbourhood of an electric circuit, is magnetised immediately. The most effective arrangement is to have a bar of very soft and pure iron acting as a core to the bobbin round which the wire is wrapped that conveys the current. The effect is nil if the current is alternating. With the usual convention concerning the direction of flow of the current, which assumes its passage to be along the wire from the copper (or corresponding portion of the battery) to the zinc, it is found that if the bar ds viewed end-on, so that the current appears to be going in a clockwise direction, the near end of the bar is made a south pole, and the more remote end a north pole. If the bar is absent the electric circuit still behaves as a magnet, but not so powerfully, and any two such circuits will attract or repel each other according to the same rules as apply with ordinary magnets.
The behaviour of the magnetic needle in pointing towards the north and south is explained by the theory that the earth itself is a magnet, somewhat irregularly magnetised, with its poles near the geographical poles. If a needle is balanced horizontally before being magnetised, it will, when rendered magnetic, tend to point in the direction of the resultant magnetic force. Thus in English latitudes it points downwards, the angle of inclination being called the dip. At the magnetic poles the dip is 90°. Isoclinic lines (q.v.) on charts of the surface of the earth mark those places where the dip has the same value. The vertical plane in which the needle tends to place itself is not usually a geographical meridian. In England, for example, the needle points about 20° west of north. Isogonic lines (q.v.) show those places where the declination from the geographical meridian has the same value. The facts mentioned above concerning the mutual actions of magnets on electric circuits or of circuits on circuits form the experimental basis of electromagnetism, by which may be explained the nature and action of dynamo-electric machinery. Modern electromagnetic theory deals particularly with the medium surrounding a magnet or an electric circuit, and suggests that not only are the stresses alike that are produced in the medium by either cause, but that they are identical in character with those that accompany the transmission of light through the medium. Electromagnetic disturbances, produced, for example, by an alternating current of high frequency in a neighbouring conductor, are transmitted through the medium at the same speed as light. This idea resulted initially from theoretical considerations of Clerk-Maxwell, but it has more recently received much support from experimental research of Hertz, who has made observations on the reflection and refraction of electromagnetic waves, which show that they are precisely similar in this respect to light-waves.