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We shall find it convenient in future to mark the poles in some definite manner, one with red and the other with blue (preferably with enamel colours). Accordingly the lines of force will be spoken of as issuing from the red pole (n) and entering the blue one (s).

17. The keeper. If a piece of soft iron, whose length is equal to the distance between the pole-pieces, is brought near to the latter, many of the lines of force leave the surrounding space, and pass through the iron. Just as chains of little iron particles were previously held fast, so now the entire mass of iron will be supported, when we arrange it as a bridge connecting the pole-pieces. On turning the magnet round, the soft iron is found to remain hanging to it, the attraction due to magnetic influences overcoming gravity. The piece of soft iron is called the keeper, because the closing of the magnetic circuit in the manner described has an important influence in preserving the properties of the magnet.

Experiment 10.-Effect of closing the magnetic circuit.-Let a magnet provided with pole-pieces be laid in a horizontal position, then form and fix the line-of-force figure just over the pole-pieces. Next place the keeper near to the pole-pieces, form another figure, bring the keeper nearer, once more form a figure, and so on. A comparison of the different figures shows how the keeper gathers together and transmits through its substance the lines of force which pass from pole to pole. The nearer it is brought to the magnet, the more numerous are the lines of force which enter and leave it, nearly all the lines passing to the end of the keeper from the nearest magnet-pole across the shallow intervening gap, where they follow approximately the shortest possible path. If the keeper is brought into actual contact with the poles, it is held fast by them.

C.-Artificial magnets of various forms and their fields

We should make but slow progress in the knowledge of magnetic phenomena if all our investigations were to be made with natural magnets, with or without pole-pieces. In

the first place because we should be limited in our choice of the most suitable shapes, and in the second place because natural magnets are always weak, and diminish in strength as time goes on. But as we know how to produce magnets of any desired shape and to strengthen their magnetic effects, we have thus the means of greatly extending our researches.

When we have described the most important forms of artificial magnets, it will be necessary to discover the distribution of their fields, which we shall investigate as before 16 by means of figures of iron filings. In the applications which we have to make of these magnets, the disposition of the lines of force is of the utmost importance.

18. Communication of magnetic properties to steel.-Iron filings, placed in the field of a natural magnet, were found in previous experiments to acquire magnetic properties which enabled them to attach themselves to other iron particles; but they lose this property as soon as they are removed from the field. It is, therefore, of the greatest importance to obtain iron in such a form that, when magnetic properties are given to it, it shall be able to retain them permanently. This is accomplished by the succession of operations by which ordinary iron is transformed into steel.

Experiment 11.-Out of a considerable number of stout steel knitting-needles, select one which, on being dipped in iron filings, retains none of them adhering to it, and is therefore non-magnetic.

If no such needle can be found, then one must be demagnetised. This is to be done by heating a feebly magnetic needle to redness, and then suddenly chilling it by immersion in cold water. The needle will then have lost its magnetism almost completely, while the steel of which it is formed will be as hard as before.

Now fix in a vertical position a soft iron wire, made specially soft by annealing, and having the same thickness and length as the needle. On bringing the pole-piece of a lodestone into contact with the upper end, the phenomenon

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already described will occur; the lower end of the wire will be able to support iron filings. On removing the pole-piece, the filings fall off again.

If the same experiment is repeated with the steel knittingneedle the lower end is able to support iron filings, but to a much less extent than was found in the case of the iron wire. But when the pole-piece is removed from the upper end of the needle, the filings do not drop off; the needle retains the property communicated to it by contact, it is permanently converted into a magnet. This communication of the magnetic property to steel is called magnetising it, the result being a so-called artificial magnet,' or 'permanent magnet.'

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This experiment shows a two-fold difference between the behaviours of iron and of steel. Iron assumes the magnetic property more readily than steel, but is also more easily demagnetised; steel, on the other hand, though harder to magnetise, retains the magnetic property which it has once acquired. The difference exhibited by iron and steel, in respect to the ease with which they change their magnetic condition, has been referred to an influence which tends to the retention of magnetism, and which has been called coercive force.' Iron has thus a small, and steel a considerable, coercive force [hysteresis].

19. Production of artificial magnets. Since in the communication of the magnetic property it is of fundamental importance to make the lines of force pass as completely as possible through the steel bar to be magnetised, more powerful magnets will be obtained when the contact is made to extend over the entire bar; that is when the bar is stroked along its entire length with one poie or with both.

(a) Single touch.-Experiment 12.-Lay on the table an unmagnetised bar of steel (for example a knitting-needle), and, holding one end firmly, stroke the needle several times along its entire length in the same direction with one pole-piece of a natural magnet, for example that which is painted red. When iron filings are strewn near the needle, they arrange themselves in chains. Lines of force leave and re-enter the needle, especially

near the ends. If the ends are dipped in iron filings, tufts of these latter will be left hanging to them.

If a steel bar has acquired magnetic properties from a natural magnet, it can further be used to magnetise another steel bar.

Experiment 13.-Stroke a knitting-needle from end to end several times in the same direction with a magnetised knittingneedle. The needle so treated will also become a magnet, and in this case, too, the magnetic activity will be manifested chiefly at the ends.

(b) Magnetisation by double touch.-On examining the lineof-force diagrams produced from bars magnetised in the manner described, it will be seen that here, as in the case of natural magnets, the lines of force issuing from one end re-enter the magnet at the other. This implies the contrariety of properties to which we have given the name 'polarity.' It will therefore be better, instead of stroking the entire length of the bar with the same pole of the magnetising magnet, to stroke one half with the pole which is painted red, and the other half with the pole which is painted blue.

Experiment 14.-A knitting-needle of the same sort as that used in experiment 12, after being tested in the manner already described, to make sure that it is not already magnetised, is held by pressing upon it at the middle with a finger of one hand, while a natural magnet provided with pole-pieces is drawn over it with the other hand, one half of the needle being stroked with one pole and the other half with the other pole, alternately, until each half has been stroked about thirty or forty times.

(c) Magnetisation by divided touch.-Since the lines of force are especially numerous between the pole-pieces of a magnet, we may also bring the two pole-pieces simultaneously into contact with the steel bar to be magnetised, and move them backwards and forwards, from end to end of the bar. This method is especially applicable when several bars are to be magnetised at the same time.

Experiment 15.-Place four or more unmagnetised knittingneedles end to end so as to form a closed figure, and let them be fixed in this position. Then let the two pole-pieces of a natural magnet be drawn repeatedly round the circuit of needles, always in the same sense. Magnetic effects will afterwards be discovered at both ends of every needle.

As regards the methods which may be used for magnetisation, the description already given must suffice; we will only remark here that all the steel bodies described in the sequel are supposed to have been strongly magnetised by one or other of these methods. We shall afterwards see that there are much more effective methods of magnetisation, depending on the employment of magnetic fields due to currents.

20. Immaterial nature of magnetism. When a body is magnetised, no change is produced in its weight.

Experiment 16.-Weigh a bar of good steel upon a sensitive balance which has no iron or steel parts. Then magnetise the bar as strongly as possible and weigh it again. The weight has remained unaltered.

This experiment must only be taken as a type of a large number which have been performed in various ways, and which have all given negative results.

It follows that when a body acquires the magnetic property, nothing of a material nature is added to it. Thus magnetisation can only consist in the rearrangement of the matter already existing in the body, or in giving to this matter some kind of motion, which constitutes the distinction between magnetised and unmagnetised bodies. The motion in question must further be of such a kind as not to involve motion of the bar as a whole in any one direction.

We shall further see how these conditions are satisfied by a cyclical or rotatory motion in the smallest parts of a magnet and of the field surrounding it, and what are essentially the transformations of energy involved in the process of magnetisation.

21. Bar magnets.-When we magnetise a steel bar of sufficient length, and having a cross-section of any form, we obtain a bar magnet. The length should be very considerably greater than the greatest measurement in any perpendicular direction. Magnetic effects are exhibited almost exclusively at the ends. Fig. 4 shows a line-of-force diagram in a horizontal plane parallel to the axis of such a magnet; from it we see that the extremities are polar

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