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. 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. [Illustration: 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.