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Page:Popular Science Monthly Volume 56.djvu/331

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317
WHAT MAKES THE TROLLEY CAR GO.

difficult, if not impossible, to obtain one without the other, although it is a simple matter to make one inactive under certain conditions. It is very generally understood that a magnet possesses the power of attraction, and that it will draw toward it pieces of iron, steel, and other magnets. The laws governing the attractive properties of magnets, however, are not so well understood, and many are not aware of the fact that under certain conditions one magnet will repel another, but such is nevertheless the case.

In Fig. 1 the lower outline, M, represents a magnet fixed in position, and the upper bar represents another magnet arranged to swing freely around the pivot a. A magnet, as is generally known, will arrange itself in a north-to-south position if suspended from its center, like a scale beam, and allowed to swing freely, and the same end will always point toward the north. On this account the ends of a magnet are called its poles, and the one that will point toward the north is designated the north pole, while the PSM V56 D0331 Diagram of magentic attraction and repulsion.pngFigs. 1, 2, 3.—Diagrams illustrating the Attraction and Repulsion of Magnets. other one is the south pole. The terms north and south poles were applied to magnets centuries ago, but at the present time the ends are more commonly designated as positive and negative. In Fig. 1 it will be noticed that the stationary magnet has its positive end upward, and this attracts the negative end of the swinging magnet. If the order of the poles is reversed, so that the positive of the swinging magnet will come opposite the positive of the stationary one, then there will be a repulsive action instead of an attraction, as is shown in Fig. 2. If the two negative ends were placed opposite, the effect would be the same. From this we see that to obtain an attraction we must place the magnets so that opposite poles come together, and that by reversing the order we obtain a repulsive action.

If the swinging magnet is replaced by a bar of iron, as is shown in Fig. 3, there will be an attraction, no matter what end of the magnet may be uppermost, thus showing that either end of a magnet will attract a bar of iron. The explanation of these different actions is that when two magnets are brought into proximity to each other each one exerts its force without any regard to the other, and if the two are set to act together they will attract one another, but if set to act in opposition they will repel. When one of the bars is not a magnet, but simply a piece of iron or steel,