Properties of Magnets - Magnetic Poles

Certain bodies, as, for instance, the iron ore called lode-stone, and pieces of steel which have been subjected to certain treatment, are found to possess the following properties, among others, and are called magnets.

If a magnet be suspended at any part of the earth's surface, except certain so-called magnetic poles, so as to be free to turn about a vertical axis, it will in general tend to set itself in a certain azimuth - i.e. with any given vertical plane, fixed in the body, inclined at a certain definite angle to the geographical meridian - and if disturbed from this position will oscillate about it.

If a piece of iron or steel, or another magnet, be brought near to a magnet so suspended, the latter will be deflected from its position of equilibrium.

If a magnet be brought near to a piece of soft iron or unmagnetised steel, the iron or steel will be attracted by the magnet.

If a long thin magnetised bar of steel be suspended so as to be capable of turning about a vertical axis through its centre of gravity, it will be found to point nearly north and south. We shall call the end which points north the north end of the magnet, the other the south end.

Definition of uniform magnetisation. - If a magnet be broken up into any number of pieces, each of these is found to be a magnet Let us suppose that the magnet can be divided into a very large number of very small, equal, similar, and similarly situated parts, and that each of the parts is found to have exactly the same magnetic properties. The magnet is then said to be uniformly magnetised.

Definition of Magnetic Axis of A Magnet. - If any magnet be supported so as to be free to turn in any direction about its centre of gravity, it is found that there is a certain straight line in the magnet which always takes up a certain definite direction with reference to the earth. This line is called the magnetic axis of the magnet.

Definition of magnetic meridian. - The vertical plane through this fixed direction is called the plane of the magnetic meridian.

Definition of magnetic poles. - If the magnet be a long thin cylindrical bar, uniformly magnetised in such a way that the magnetic axis is parallel to the length of the bar, the points in which the axis cuts the ends of the bar are the magnetic poles. The end of the bar which tends to point north, when the magnet is freely suspended, is the north, or positive pole; the other is the south or negative pole. Such a magnet is called solenoidal, and behaves to other magnets as if the poles were centres of force, the rest of the magnet being devoid of magnetic action. In all actual magnets the magnetisation differs from uniformity. No two single points can strictly be taken as centres of force completely representing the action of the magnet For many practical purposes, however, a well-made bar magnet may be treated as solenoidal with sufficient accuracy; that is to say, its action may be regarded as due to two poles or centres of force, one near each end of the magnet.

The following are the laws of force between two magnetic poles:

(1) There is a repulsive force between any two like magnetic poles, and an attractive force between any two unlike poles.

(2) The magnitude of the force is in each case numerically equal to the product of the strength of the poles divided by the square of the distance between them.

This second law is virtually a definition of the strength of a magnetic pole.

In any magnet the strength of the positive pole is equal in magnitude, opposite in sign, to that of the negative pole. If the strength of the positive pole be m, that of the negative pole is -m. Instead of the term 'strength of pole', the term 'quantity of magnetism' is sometimes used. We may say, therefore, that the uniformly and longitudinally magnetised thin cylindrical bar behaves as if it had quantities m and -m of magnetism at its two ends, north and south respectively; we must, however, attach no properties to magnetism but those observed in the poles of magnets. If, then, we have two magnetic poles of strengths m and m', or two quantities of magnetism m and m', at a distance of r centimetres apart, there is a force of repulsion between them which, if m and m' are measured in terms of a proper unit, is

mm'/r² dynes

If one of the two m or m' be negative, the repulsion becomes an attraction.

The C.G.S. unit strength of pole is that of a pole which repels an equal pole placed a centimetre away with a force of one dyne.

In practice it is impossible to obtain a single isolated pole; the total quantity of magnetism in any actual magnet, reckoned algebraically, is always zero.