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Figure 44 - Magnetic lines of force around a bar magnet

In order to represent the strength of the magnetic field, there is a well-established convention concerning the number of lines of force to be drawn per unit area. The agreement is that the number of lines of force per square centimeter shall be just equal to the number of dynes with which the field would act on a unit pole. For example, a magnetic field equal to 10 oersteds is represented by drawing 10 lines of force per square centimeter.

Molecular Theory of Magnetism. - It is assumed that every sub­stance which is capable of being magnetized consists of a very large number of molecular magnets, probably no larger than the molecules out of which the substances are made. When the substance is unmagnetized, these molecular magnets are not arranged in any particular direction, but are oriented indiscriminately. When the substance is magnetized, a larger number of them are made to point along the axis of the magnet than point in another direction. In the interior of the magnet, the little north poles lie so close to the little south poles that each destroys the influence of the other. At one end of the bar are tree north poles, and at the other end are free south poles. The sum of all these little north poles makes the N pole of the magnet, and the sum of all the little south poles makes the S pole of the magnet.

Magnetic Induction. - When a bar magnet is placed near a piece of unmagnetized iron, the elementary magnets in the iron tend to arrange themselves so that all the N poles point in one direction and all the S poles in the opposite direction. The piece of iron thus becomes a magnet so long as it is in the presence of the permanent bar magnet. South-seeking poles are produced near the north-seeking pole of the bar magnet and north-seeking poles at the other end of the piece of soft iron. This process of magnetization is known as magnetization by induction.

Molecular State of a Magnetized Body. - It is found that when a magnet is broken in two, two complete magnets result, two new poles appearing at the fracture. These poles must have existed in the original magnet, but without producing external effects, since they neutralized each other.

Hence magnetization is a state existing everywhere in a magnet, but manifested only at the poles. On breaking the magnet into still shorter pieces, we still get complete magnets. While the subdivision cannot actually be carried to parts of the size of molecules, there are strong reasons for believing that the molecules (or atoms) of a magnetic substance are magnets.

We can now explain what takes place when a rod of iron is magnetized. The molecular magnets in the unmagnetized rod were like a lot of small magnets thrown into a box, their axes being turned in all directions. In magnetizing the rod we twist the molecular magnets around so that their axes are more or less parallel to the length of the rod, a process that can be imitated by using a glass tube filled with iron filings. The more completely they are lined up in this direction, the stronger is the resulting magnet.

In this position they possess potential energy which they had not before, and this came from the work we had to do to turn them. It is found that there is a limit to the strength to which a rod can be magnetized by using stronger and stronger inducing magnets. All the molecular magnets then agree in direction and the rod is described as saturated.

Hammering, bending, or twisting an iron rod when it is near a magnet increase its magnetization, and they also tend to demagnetize a permanent magnet, owing to the agitation of the molecules that they produce. A permanent magnet can also be demagnetized by heating it to a red heat, since heat is in itself a molecular disturbance and tends to destroy the alinement of the molecules. For the same reason soft iron cannot be magnetized temporarily when it is at a red heat. A long wire of soft iron mounted to swing will cling to the pole of a magnet, but it loses its hold when heated by a Bunsen burner.

When out of the flame it again becomes magnetic and returns to the magnet, and so it continues to vibrate. A similar experiment with a strip of nickel attached to a blackened copper disk (to promote cooling) succeeds when the source of heat is merely an alcohol flame, since nickel loses its magnetic properties at a much lower tem­perature than iron. The critical temperature is called the Curie point [2, C. 108 - 111].

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