MAGNETISM. 



agencies of opposite kinds, capable, 

 when separate, of acting with great 

 energy, but being, when combined toge- 

 ther, perfectly neutralized and exhibit- 

 ing no sign of activity. As there were 

 two electrical powers, the positive and 

 the negative, or, as some prefer denomi- 

 nating them, the vitreous and the re- 

 sinous, so there are two magnetic 

 powers, distinguished as the northern 

 and the southern polarities ; or, as some 

 choose to designate them, the austral 

 and the boreal. The parallel is most 

 exact. Both sets of phenomena are go- 

 verned by the same characteristic law, 

 which may be expressed by the follow- 

 ing concise and general formula, namely, 

 between like powers there is repul- 

 sion ; between unlike, attraction. 



4. Induction. 



(16.) The communication of mag- 

 netic properties to iron or steel by the 

 mere approach of the poles of a magnet, 

 is also analogous in its principal cir- 

 cumstances to electric induction. 



Fig. 5. 



If the north pole, N, of a magnet A 

 (fig. 5), be brought near to the end s of 

 an unmagnetized bar of iron, B, that 

 end will immediately acquire the pro- 

 perties of a south pole, while the oppo- 

 site, or distant end, n, will at the same 

 time be converted into a north pole. If, 

 instead of the north pole, N, the south 

 pole, S, had been presented to the bar, 

 the changes effected in B would have 

 been just the reverse ; the adjacent end 

 would have acquired the northern, and 

 the distant end the southern polarity. 



(17.) Thus we may observe that each 

 pole of a magnet induces the opposite 

 kind of polarity in that end of the iron 

 which is nearest to it, and the same kind 

 on the remotest end ; just as happens in 

 the induction of electricity, in which the 

 positive state induces the negative, and 

 the negative the positive state, in those 

 parts of an insulated conductor which 

 are nearest to the electrified body ; and 

 a similar state of electricity in the dis- 

 tant end. 



(18.) That the iron, while it remains 

 in the vicinity of the magnet, possesses 

 the magnetic properties, may be shown 

 by a variety of experiments. 



First, it attracts other iron. If we 

 take, for instance, a key (fig. 6), and 



Fig. 6. 



hold it horizontally near one of the poles 

 of a strong magnet, also lying in a hori- 

 zontal position, but not touching the 

 key ; and if we then apply another light 

 piece of iron, such as a small nail, to the 

 other end of the key, the nail will hang 

 from the key, and will continue to do so 

 while we slowly withdraw the magnet 

 horizontally from the key. When the 

 magnet has been moved beyond a cer- 

 tain distance, the nail will drop from the 

 key, because the magnetism induced on 

 the key becomes, at that distance, too 

 weak to support the weight of the nail. 

 That this is the real cause of its falling 

 off may be proved by taking a still 

 lighter fragment of iron, such as a piece 

 of very slender wire, and applying it to 

 the key. The magnetism of the key 

 will still be sufficiently strong to support 

 the wire, though it could not support 

 the nail : and it will continue to support 

 the wire, even when the magnet is yet 

 further removed; at length, however, 

 when the distance is still greater, the 

 wire, in its turn, drops off. 



The same effects may be observed if 

 the nail be placed in contact with the 

 near end of the key ; but they are gene- 

 rally less distinct, on account of the di- 

 rect influence which the magnet exerts 

 on the nail, and which interferes in some 

 degree with the action of the key, so 

 that the results become complicated. 



(19.) The same series of phenomena 

 take place when the key is held above 

 or below the pole of the magnet, or on 

 either side of it. The key will hold the 

 nail or wire suspended from either end, 

 as long as the magnet is near enough to 

 exert sufficient influence on the key. 



(20.) If the key be laid upon a piece 

 of paper on a table, and several small 

 bits of wire, or iron filings, be strewed 

 round one end of the key, which we sup- 

 pose to be devoid of all magnetic pro- 

 perties, no adhesion will be perceptible 

 between the fragments of iron and the 

 key. Let us now approach the pole of a 

 magnet to the other end of the key ; we 

 immediately observe the filings and 

 lighter pieces of iron spontaneously, and 

 of one accord, move towards the key, and 



