Fig. 13.—Horse-shoe and Bar Magnets, with Keepers.
If we roll a bar magnet amongst iron filings we find that the filings remain clinging to it in two tufts, one at each end, and that few or none adhere to the middle. These two points towards which the filings are attracted are called the poles of the magnet. Each pole attracts filings or ordinary needles, and one or two experiments will show that the attraction becomes evident while the magnet is still some little distance away. If, however, we test our magnet with other substances, such as wood, glass, paper, brass, etc., we see that there is no attraction whatever.
If one of our bar magnets is suspended in a sort of stirrup of copper wire attached to a thread, it comes to rest in a north and south direction, and it will be noticed that the end which points to the north is marked, either with a letter N or in some other way. This is the north pole of the magnet, and of course the other is the south pole. If now we take our other magnet and bring its north pole near each pole of the suspended magnet in turn, we find that it repels the other north pole, but attracts the south pole. Similarly, if we present the south pole, it repels the other south pole, but attracts the north pole. From these experiments we learn that both poles of a magnet attract filings or needles, and that in the case of two magnets unlike poles attract, but similar poles repel one another. It will be noticed that this corresponds closely with the results of our experiments in [Chapter I]., which showed that an electrified body attracts unelectrified bodies, such as bits of paper or pith balls, and that unlike charges attract, and similar charges repel each other. So far as we have seen, however, a magnet attracts only iron or steel, whereas an electrified body attracts any light substance. As a matter of fact, certain other substances, such as nickel and cobalt, are attracted by a magnet, but not so readily as iron and steel; while bismuth, antimony, phosphorus, and a few other substances are feebly repelled.
The simplest method of magnetizing a piece of steel by means of one of our bar magnets is the following: Lay the steel on the table, and draw one pole of the magnet along it from end to end; lift the magnet clear of the steel, and repeat the process several times, always starting at the same end and treating each surface of the steel in turn. A thin, flat bar of steel is the best for the purpose, but steel knitting needles may be made in this way into useful experimental magnets.
We have seen that a magnet has two poles or points where the magnetism is strongest. It might be thought that by breaking a bar magnet in the middle we should get two small bars each with a single pole, but this is not the case, for the two poles are inseparable. However many pieces we break a magnet into, each piece is a perfect magnet having a north and south pole. Thus while we can isolate a positive or a negative charge of electricity, we cannot isolate north or south magnetism.
If we place the north pole of a bar magnet near to, but not touching, a bar of soft iron, as in [Plate II.a], we find that the latter becomes a magnet, as shown by its ability to support filings; and that as soon as the magnet is removed the filings drop off, showing that the iron has lost its magnetism. If the iron is tested while the magnet is in position it is found to have a south pole at the end nearer the magnet, and a north pole at the end farther away; and if the magnet is reversed, so as to bring its south pole nearer the iron, the poles of the latter are found to reverse also. The iron has gained its new properties by magnetic induction, and we cannot fail to notice the similarity between this experiment and that in [Fig. 2], [Chapter II]., which showed electro-static induction. A positively or a negatively electrified body induces an opposite charge at the nearer end, and a similar charge at the further end of a conductor, and a north or a south pole of a magnet induces opposite polarity at the nearer end, and a similar polarity at the further end of a bar of iron. In [Chapter II]. we showed that the attraction of a pith ball by an electrified body was due to induction, and from what we have just learnt about magnetic induction the reader will have no difficulty in understanding why a magnet attracts filings or needles.
PLATE II.
(a) EXPERIMENT TO SHOW MAGNETIC INDUCTION.
