401. Attraction and Repulsion in Magnets.—You have seen in induction that in magnets like poles repel while unlike attract. But this law can be more strikingly illustrated. If a magnet be placed on a pivot, as in Fig. 280, and another magnet be brought near it, attraction or repulsion will be manifested according to the mode of presentation. If a north pole be presented to a north pole, or a south to a south, repulsion will be the result. But if a north pole be presented to a south, or a south to a north, then attraction will be manifested.
Fig. 282.
402. Magnetic Curves.—The polarity of magnetism causes a very singular arrangement of iron filings when gently agitated upon a sheet of paper over a magnet, as represented in Figure 282. The curves which you see have been supposed by some to be occasioned by the escape of some fluid or influence from the magnet in these particular directions. But they are owing entirely to the fact that each bit of filing is polarized by the bit next preceding it in the row reckoning from the magnet outward, the nearest one in each row deriving its magnetic state from the magnet itself. This being so, as the chief power resides in the ends of the magnet, it is easy to see how such a disposition of the lines of magnetic filings is effected. These curves may be beautifully and curiously varied by having several magnets variously arranged under the paper.
403. Artificial Magnets.—The power residing in the loadstone can be communicated readily, as you have seen, to iron and steel. Though soft iron takes the magnetic influence more readily than steel, it does not retain it as steel does, and the latter is therefore used in making artificial magnets. When a magnet imparts its magnetic influence it loses none of its own power, whether it be an original loadstone or an artificial magnet. There, are many ways of imparting magnetism permanently to steel, but I will notice only two of them. If you wish to magnetize a bar or needle pass one pole of a magnet from one end of it to the other a considerable number of times, always in the same direction. A more effectual way is to take two magnets, and, placing the south pole of one and the north pole of the other in contact over the middle of the bar or needle, draw them slowly and steadily apart toward the opposite ends. This process must be repeated several times.
Fig. 283.
Fig. 284.
404. Horseshoe Magnets.—One of the most common forms of the magnet is the horseshoe magnet, Fig. 283. There is a piece of soft iron attached to the end of this, held there by attraction. This is called the armature. So long as it is suffered to remain there it is a magnet having its two poles, the north pole + being attached to the south pole - of the magnet which holds it, while the reverse is the case with its south pole. The object of the armature is to preserve the power of the instrument. Indeed it is found that the exertion of the magnet's power not only preserves but actually increases it. If you attach, therefore, to a magnet an armature having a hook, as seen in Fig. 284, you can add to the weight gradually from day to day, and so considerably augment the power of the magnet.