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. [Illustration: PLATE II. (_a_) EXPERIMENT TO SHOW MAGNETIC INDUCTION.] [Illustration: (_b_) EXPERIMENT TO SHOW THE PRODUCTION OF MAGNETISM BY AN ELECTRIC CURRENT.] Any one who experiments with magnets must be struck with the distance at which one magnet can influence filings or another magnet. If a layer of iron filings is spread on a sheet of paper, and a magnet brought gradually nearer from above, the filings soon begin to move about restlessly, and when the magnet comes close enough they fly up to it as if pulled by invisible strings. A still more striking experiment consists in spreading filings thinly over a sheet of cardboard and moving a magnet to and fro underneath the sheet. The result is most amusing. The filings seem to stand up on their hind legs, and they march about like regiments of soldiers. Here again invisible strings are suggested, and we might wonder whether there really is anything of the kind. Yes, there is. To put the matter in the simplest way, the magnet acts by means of strings or lines of force, which emerge from it in definite directions, and in a most interesting way we can see some of these lines of force actually at work.