attraction over the repulsion, that is to say, being proportional

363. Magnetic Induction.—While the mutual action of magnets may be compared to that of electrified bodies upon each other, that of a magnet upon a piece of soft iron may be compared in some respects to the action of an electrified upon a neutral body.

We may regard the magnet as decomposing the neutral magnetic fluid of the soft iron, in a manner like that in which an electrified substance decomposes the neutral fluid of the body near it. Thus if the marked pole A of a large magnet be presented to a bit of soft iron, the soft iron becomes temporarily a magnet, having its unmarked pole b next the marked pole A of the large magnet, while the marked pole a of the small bit of iron will be at the extremity farthest from A. The attraction of A for b will therefore be greater than its repulsion for a, and hence the bit of iron will be bodily attracted to the large magnet, and will probably fly towards it, and attach itself to its extremity or pole. This is the reason why iron filings attach themselves to magnets, and a bundle of nails or even a heavy iron weight may be held up in this manner by a very powerful magnet.

364. Effect of breaking a Magnet.—It might be supposed that if we broke a magnet A B, we should get two magnets, one containing only the marked pole A, and the other only the unmarked pole B of the original magnet, just as in the experiment of Art. 343 we separated the two electricities of the neutral conductor. Such, however, is by no means the case; for when we break a large magnet into two, it immediately forms two small complete magnets. Thus if we break a magnet A B (Fig. 121) at its centre, a pole B' is immediately formed to the left hand of the point of rupture, and a pole A' to the right, so that A B′ and A' B are two complete magnets.

Thus by breaking a magnet into a number of pieces we make so many separate magnets; and in order to explain

this, it has been supposed that all the magnetism of one kind is not concentrated in the marked pole of a magnet, and all that of the opposite kind in the unmarked pole, but

that in each particle throughout the body of the magnet there is a separation between the two magnetisms, so that the state of things in a magnet may be exhibited by Fig. 121.

Now if a piece of soft iron be brought into the neighbourhood of this arrangement at the left, the marked pole (a) of each particle of the magnet will be somewhat nearer the soft iron than the unmarked pole (b), and the sum of all the small effects upon the soft iron will be to the same effect as if all the magnetism corresponding to the marked pole were concentrated in the left-hand end of the magnet, and all the opposite magnetism in the other end. Also we see at once that if a magnet consist of an arrangement of this kind, and if it be broken into two parts, each part will become a separate magnet precisely similar to the whole.

365. How to make Magnets.-When soft iron is made to approach a magnet, temporary magnetic poles are readily induced in the soft iron, so that it becomes a magnet for the time being, and while attracted to the magnet, it will attract another piece of soft iron, just as the magnet itself would. Nevertheless all these properties are lost as soon as the soft iron is removed. We are therefore justified in saying that soft iron cannot retain magnetism, and that we cannot have a permanent magnet made of this substance. Hard steel is the substance that can be best made into a permanent magnet.

We may magnetize a steel bar by the following process :

Let B′′ A′′ (Fig. 122) be the bar which we wish to magnetize, and let B A, B' A' be two powerful magnets, A being the marked pole of the one, and B' the unmarked pole of the

B

A B

B"

FIG. 122.

other. Bring the two magnets together, as in the figure, at the centre of the bar to be magnetized, then simultaneously draw them along the bar towards the extremities, moving A towards B" and B' to A". Repeat this process several times, and it will be found that the bar has become a magnet, with A" for its marked, and B′′ for its unmarked pole.

366. Effect of Heat on Magnets.-When a magnet is slightly heated it loses part of its magnetism, which is mostly recovered when it is again cooled to its original temperature; but if it be heated beyond a certain limit, the loss of magnetism will not be recovered when it cools, and if heated to redness it loses all trace of magnetic properties of any kind. Soft iron also, when heated to redness, loses the property of being attracted by a magnet.

A similar limit exists in the case of the other magnetic metals, nickel and cobalt, which if heated sufficiently will ultimately lose their magnetic properties.

367. The Earth acts as a Magnet.—If a magnetic needle be suspended horizontally it will point in this country in a direction nearly north and south, the marked pole being about 20° to the west of the north. A vertical plane passing through the poles of such a needle is called the magnetic meridian.

Again, if a truly balanced needle be suspended by a delicate horizontal axle, so placed that the magnet moves in the plane of the magnetic meridian, the marked pole will dip downwards, until the needle makes an angle with the

horizon of about 68°. We are therefore justified in saying that were a magnetic needle perfectly free to place itself as it chose, it would be found in a vertical plane passing 20° to the west of north, and with its marked pole dipping downwards and making an angle of 68° with the horizon. The earth, in fact, acts like a gigantic magnet, of which the unmarked pole lies to the north, and the marked pole to the south, in consequence of which the marked pole of a freely suspended needle points in this country approximately to the north.

It is this property that makes the magnetic needle of such value to mariners, who might not otherwise know in what direction to steer.

But the marked pole of a needle will not everywhere and always point as it does in Great Britain at the present moment. Two hundred years ago the needle pointed to the true geographical north in London, while now it points 20° to the west of north. Again, if we travel far to the north we come to a place that is called the magnetic pole, where the needle, if free to place itself as it chose, would point with its marked end vertically downwards; at this place also, were it horizontally balanced, it would have no tendency to turn in one direction more than another; in fact, the unmarked magnetic pole of the earth being there directly under our feet, the needle merely points with its marked pole vertically downwards, and is thus of no use to the mariner.

In like manner, if we travel far south we shall come to the earth's marked pole, where the unmarked end of our needle will point vertically downwards, and where a horizontally. suspended needle will settle in any direction. It is unknown in what manner the earth acquired its magnetism, but it has been discovered by Sir E. Sabine that the magnetic properties of the earth are in some way connected with the spots which appear from time to time on the surface of the sun, so that in those years when there are most spots, there are most disturbances of the magnetism of the earth.

We ought to mention that the effect of the earth's mag

netism upon a magnetic needle is merely directive; that is to say, the earth twists round a freely suspended needle, but does not attract it bodily otherwise than it does any ordinary non-magnetic substance. The reason is, that the magnetic pole of the earth is very far removed from the poles of the needle, so that the attraction of the earth's pole for the one pole of the needle is not sensibly greater than its repulsion for the other, and therefore the needle is not bodily attracted towards the earth in virtue of its being a magnet.

It will be shown in the following pages that magnetism is very intimately associated with electricity in motion.

LESSON XLI.--VOLTAIC BATTERIES.

368. In the year 1786, Galvani, Professor of Anatomy in Bologna, remarked that convulsions were produced in the leg of a frog when the lumbar nerves were connected with the crural muscles by means of a circuit composed of the two metals iron and copper, and he attributed the effect to electricity inherent in the animal. Shortly afterwards the subject was taken up by the celebrated Volta, who came to the conclusion that the source of electricity in Galvani's experiment was the contact of two heterogeneous metals; and he was soon led by this view to construct a pile, which is the origin of the Galvanic or Voltaic batteries of the present day.

His pile was of very simple construction. He built on an insulated foundation of glass or resin a number of discs placed in the following order :—a disc of copper, one of zinc, and one of cloth or flannel, moistened with acidulated water; then copper, zinc, cloth, as before.

Now in such an insulated pile it may be shown, by means of the electroscope, that the lower half is charged with negative, and the upper half with positive electricity, and that the tension is most powerful at the extremities, so that the lower copper plate is decidedly negative, and the upper zinc plate decidedly positive in its indication. If now the