The Magnet

Teacher’s Story

NTIL comparatively recent times, the power of the magnet was so inexplicable that it was regarded as the working of magic. The tale of the Great Black Mountain Island magnet described in the “Arabian Nights Entertainments”—the story of the island that pulled the nails from passing ships and thus wrecked them—was believed by the mariners of the Middle Ages. Professor George L. Burr assures me that this mountain of lodestone and the fear which it inspired were potent factors in the development of Medieval navigation. Even yet, with all our scientific knowledge, the magnet is a mystery. We know what it does, but we do not know what it is. That a force unseen by us is flowing off the ends of a bar magnet, the force flowing from one end attracted to the force flowing from the other and repellent to a force similar to itself, we perceive clearly. We also know that there is less of this force at a point in the magnet half-way between the poles; and we know that the force of the magnet acts more strongly if we offer it more surface to act upon, as is shown in the experiment in drawing a needle to a magnet by trying to attract it first at its point and then along its length. That this force extends out beyond the ends of the magnet, the child likes to demonstrate by seeing across how wide a space the magnet, without touching the objects, can draw to it iron filings or tacks. That the magnet can impart this force to iron objects is demonstrated with curious interest, as the child takes up a chain of tacks at the end of the magnet; and yet the tacks when removed from the magnet have no such power of cohesion. That some magnets are stronger than others is shown in the favorite game of “stealing tacks,” the stronger magnet taking them away from the weaker; it can also be demonstrated by a competition between magnets, noting how many tacks each will hold.

One of the most interesting things about a magnet is that like poles repel and opposite poles attract each other. How hard must we pull to separate two magnets that have the south pole of one against the north pole of the other! Even more interesting is the repellent power of two similar poles, which is shown by approaching a suspended magnetized needle with a magnet. These attractive and repellent forces are most interestingly demonstrated by the experiment in question 13 of the lesson. These needles floating on cork join the magnet or flee from it, according to which pole is presented to them.

Not only does this power reside in the magnet, but it can be imparted to other objects of iron and steel. By rubbing one pole of the magnet over a needle several times, always in the same direction, the needle becomes a magnet. If we suspend such a needle by a bit of thread from its center, and the needle is not affected by the nearness of a magnet, it will soon arrange itself nearly north and south. It is well to thrust the needle through a cork, so it will hang horizontally, and then suspend the cork by a thread. The magnetized needle will not point exactly north, for the magnet poles of the earth do not quite coincide with the poles of the earth’s axis.

The direction assumed by the magnetized needle may be explained by the fact that the earth is a great magnet, but the south pole of the great earth magnet lies near the North Pole of the earth. Thus, a magnet on the earth’s surface, if allowed to move freely, will turn its north pole toward the south pole of the great earth magnet. Then, we might ask, why not call the earth’s magnetic pole that lies nearest our North Pole its north magnetic pole? That is merely a matter of convenience for us. We see that the compass needle points north and south, and the arm of the needle which points north we conveniently call its north pole.

The above experiment with a suspended needle shows how the mariner’s compass is made. This most useful instrument is said to have been invented by the Chinese, at least 1400 B. C., and perhaps even longer ago. It was used by them to guide armies over the great plains, and the needle was made of lodestone. The compass was introduced into Europe about 1300 A. D., and has been used by mariners ever since. To “box the compass” is to tell all the points on the compass dial, and is an exercise which the children will enjoy.

We are able to tell the direction of the lines of force flowing from a magnet, by placing fine iron filings on a pane of glass or a sheet of paper and holding close beneath one or both poles of a magnet; instantly the filings assume certain lines. If the two ends of a horseshoe magnet are used, we can see the direction of the lines of force that flow from one pole to the other. It is supposed that these lines of magnetic force streaming from the ends of the great earth magnet cause the Northern Lights, or Aurora Borealis.

Lodestone is a form of iron with a special chemical composition, and it is a natural magnet. Most interesting stories are told of the way the ancients discovered this apparently bewitched material, because it clung to the iron ends of their staffs or pulled the iron nails from their shoes. In the Ward’s collection of minerals sent out to schools, which costs only one dollar, there is included a piece of lodestone, which is of perennial interest to the children.

Magnets made from lodestone are called natural magnets. A bar magnet or a horseshoe magnet has received its magnetism from some other magnet or from electrical sources. An electro magnet is of soft iron, and is only a magnet when under the influence of a coil of wire charged with electricity. As soon as the current is shut off the iron immediately ceases to be a magnet.