It is found that when iron or steel is raised to a white heat, it is not attracted by a magnet; in fact, it ceases to be magnetic. This phenomenon has not been satisfactorily explained by any theory; and, until further investigation has been made, we cannot say whether it makes for or against the above molecular theory.

Experiments to illustrate molecular theory.—(i.) We can magnetise a tube of steel filings, and then render it neutral by shaking it up. As the filings are not free to traverse the tube, but merely turn about where they are, this illustrates the case of neutrality by the pairing off of the molecules.

(ii.) Magnetise a strip of watch-spring, and test it.

If this watch-spring be now made into a complete circuit, we get almost complete external neutrality; while the evident magnetism is again restored when we break the circuit.

(iii.) A poker may be magnetised by being held along the lines of the earth's magnetism (see Chapter III. § 8) and in that position hammered.

(iv.) The magnetism of a steel knitting-needle may be reversed if it is held with south-seeking end downwards along the lines of earth's magnetism, and in that position kept at a bright red heat for some time; being then allowed to cool in the same position.

(v.) It was shown by Joule that a bar when magnetised increases in length. This might well be the case if each pair of molecules (see fig. ii. above) does in fact rotate from a parallel, to an end-on, position.

(vi.) A tube is filled with water in which is suspended magnetic oxide of iron in a finely divided condition; the whole being nearly opaque to light.

If the tube be magnetised in the direction of its length, it is found that it is now less opaque to light in this direction. This would tend to show that the particles of the magnetic oxide have arranged themselves end-on in the direction of magnetisation.

§ 6. Induction, General Phenomena.-The matter of induction will be clearer to the learner when he has learnt something of Fields of Force,' in Chapter II.

At present we shall only describe the actual facts observed.

When a piece of iron is placed near the pole of a magnet, the end of the iron that is nearest the pole acquires a polarity opposite to that of the said pole; while the end of the iron that is furthest away acquires a polarity similar to that of the magnetpole.

This action of a magnet, in making iron near it also magnetic, is called 'Induction.'

It is more powerful as the 'inducing' magnet is more powerful, and as the iron is nearer to the magnet.

It is stronger and less permanent as the 'coercive force' of the iron is small (or the molecular freedom great), weaker and more permanent as the 'coercive force' is greater.

Experiments in Induction.—(i.) NS is a powerful magnet; and ns, n's, n′′s", &c., are pieces of very soft iron arranged near NS as shown. The

letters of the accompanying figure indicate the observed polarities of the pieces of soft iron. One may test these polarities either by means of a small compass-needle applied to the ends of the iron pieces, or in some such way as that indicated in the next experiment.

(ii.) The writer believes that the following experiment is due to Professor Guthrie.

NS is a powerful permanent magnet and vo is a bar of soft iron hung above NS, parallel and close toit. Then vs will be acted upon inductively, and will temporarily become a magnet, its poles being atv and σ.

If a north seeking pole N' be approached to the end v which lies above the south-seeking pole S, it S is found that N' repels v. Hence vis north-seeking, or S has induced atv a polarity opposed to its own.

This is, in fact, indicated by the simple fact that vo will, if displaced, return to the position represented in the figure.

We may say generally that induction takes place through any solid, liquid, or gas; it making no perceptible difference what substance is interposed between the magnet and the iron, unless we are making very delicate observations indeed.

There is, however, one very important exception. A sheet of perfectly soft iron, or iron with absolutely zero 'coercive force,' would act as a perfect screen; and no induction would take place on a piece of iron or steel suspended in a closed vessel made of such soft iron. To a greater or less degree, according to its 'softness,' all iron and steel will act as a screen.

We have spoken hitherto of iron and steel only. There are,

however, other bodies, notably cobalt, nickel, and manganese, which possess to a certain degree magnetic properties. More will be said of these in Chapter XX.; and it will there be pointed out how feebly magnetic are these bodies when compared with iron or steel.

§ 7. Use of Keepers.-When a bar is magnetised, there are at the one end a set of molecules with their north-seeking poles 'free,' and at the other end a corresponding set with their southseeking poles free.' This condition of things is, as we have said before, unstable; and there is in a magnetised bar a tendency for the molecules to pair off again in such a way as to give neutrality of the nature indicated in § 5, fig. ii. This tendency is resisted by molecular rigidity, or coercive force; but a loss of magnetisation is inevitable if the bar be subjected to molecular disturbance of any kind.

But if we can, by means of soft iron pieces applied to the magnet, make the molecular circuit complete, we shall obtain a

stable molecular arrangement similar to that of § 5, fig. iii. The whole system will give but little signs of external magnetism until the soft iron pieces are removed. The accompanying figures show how a horseshoe magnet, or a pair of bar magnets, may be provided with soft iron keepers.

The letters indicate the polarities temporarily induced in these

keepers. The molecular arrangements, similar to that of $ 5, fig. iii., can readily be imagined.

§ 8. Methods of Magnetisation.-In making a magnet we must have several points in view.

(i.) The material must be one having great molecular rigidity or coercive force; so that the rearrangement of molecules, to which is due the external or evident magnetism, may be retained. Hence we use hard steel.

(ii.) During the process of magnetisation the molecules should be given as free play as possible. Hence, hammering, twisting, and other mechanical aids to molecular freedom may be employed if convenient. It is found that repetitions of the magnetising process, even when each action first undoes the work of the preceding action before redoing it, give better final results than a single application of the process.

(iii.) We must apply as powerful a magnetising influence as we can. The more powerful the 'field' (see Chapter II.) in which we place our bar, the more will the molecules be rotated from their 'short-circuited' position into the position giving most powerful external magnetism.

I. Single Touch.-Here a magnet is simply drawn from end to end of the bar in the position shown in the figure. This 'stroking' process is repeated many times.

Here we can imagine the molecules rotating, as the inducing magnet pole moves down the bar.

S

They will always, e.g., turn their south poles towards a north inducing magnet pole. Hence we should predict that finally the end last touched by a north inducing pole will be left of south polarity, while that first touched will be left of north polarity. Each time that the 'stroke' is repeated it is clear that the molecular arrangement due to the preceding 'stroke' will be undone, or at least seriously disturbed. It seems, therefore, at first sight somewhat strange that there is so much gained by repetition. Probably the explanation is that this repetition gives greater freedom of movement to the molecules; and hence the final stroke has greater effect than had the first.

n

FIG. i.

II. Double Touch.-Here two magnets are used; they are

placed, N pole to S pole, in an inclined position, as shown. We begin the stroke in the middle of the bar, and we move the magnets up to one end, back to the other end, and then up to the middle again; this process being repeated many times. The ends of the bar may also be placed on the opposite poles of two magnets. This increases the effect.

III. Separate Touch.-Or again the two inducing magnets may be drawn away from each other to the two ends respectively, and brought back to the middle by being lifted in a wide curve through the air; and this process repeated.

IV. By an Electric Current -But by far the most powerful method is magnetisation by means of an electric current. Of this we shall say much hereafter in Chapter XX.