397. Loadstones.—It was discovered many centuries ago that a certain ore of iron has the property of attracting pieces of common iron or of steel. The fact was probably considered at first as a mere curiosity, and the world were slow to find out its value. It is not till quite recently that it has been discovered that in magnetism we have one of the great forces of the earth; and even now we know but little probably of the real extent and variety of its action. New and important discoveries are yet undoubtedly to be made in regard to the agency and the laws of this mysterious power, and its connections with the other grand forces of nature. The terms magnet and magnetism come from the fact that the loadstone was first found near Magnesia, an ancient city in Asia Minor. This ore appears in considerable masses in the iron mines of Sweden and Norway, and also in different parts of Arabia, China, and Siam. It has occasionally been found in small quantities in England and in this country.
398. Attraction of Magnetism.—The attraction of the magnet and iron for each other is exhibited in many different ways. If a magnet be brought near to a heap of iron-filings or needles, it will have a quantity of them adhering to it as you raise it up. In the toy fishes of children there is fastened in the head a bit of iron, which occasions the following of the fishes after the magnet. In this case you can see very plainly that the nearer the magnet and the iron are to each other the stronger is the attraction. Indeed, the attractive influence is governed by the same law in regard to distance as the common attraction of matter is, viz., it is inversely as the square of the distance. The attraction also is mutual here, the iron attracting the magnet as much as the magnet does the iron.
Fig. 279.
Fig. 280.
399. Poles of the Magnet.—Every magnet has two poles. It is about these poles where the chief power resides. For this reason, if a magnet be rolled in iron-filings, these are collected about the ends, as represented in Fig. 279. There is a diminution of attraction from the ends to the middle line, which is called the neutral line. These poles are called north and south poles, because if a magnet be suspended, or be supported upon a pivot, so that it can revolve, it will take a north and south direction, one of its ends invariably pointing toward the north. In Fig. 280 is represented a magnet supported upon a pivot, C.
Fig. 281.
400. Magnetism by Induction.—The magnet in exerting its attraction really temporarily makes a magnet of what it attracts. Actual contact is not necessary to this result. Thus if a large key be only brought very near to a powerful magnet it will support small keys, as represented in Fig. 281. When the key is removed away from the magnet the keys attached to it fall. You see the analogy to the induction of electricity noticed in § 379. As in the induction of electricity, so here the two ends of the body in which the influence is induced are in opposite states. If the end of the magnet, to which the first key is near or attached, be the north pole, the end of the key next to the magnet is the south pole, and its farther end is the north pole. The same is the case with the small key attached to the end of the large one. And so if a nail should hang from the small key, and a needle from that, both of these would have the same polarities. But all this would be reversed if the large key were attached to the south pole of the magnet. In this case the upper end of each of these articles would be the north pole, and its lower end the south pole.