MAGNETISM AND ELECTRICITY. 141 
pole of the former and repels the other. Thus, calling the two poles of the 
first magnet, n, s, and those of the second, n’,s’, then n’ will attract s and 
repel z, while s’ will attract m and repel s. And, in general, the like or 
corresponding poles of any two magnets will repel, while the unlike will 
attract each other. The two poles of the same magnet are therefore said 
to be unlike or opposite. Instead, then, of one magnetic fluid, the 
existence of two may be conveniently assumed, one acting at each 
pole. 
Iron, under the influence of a magnet, itself becomes magnetic, possessing 
a like power of attraction, as is seen in the case of iron filings in the 
experiments previously adduced. If a small cylindrical rod of iron be 
appended to a magnet (fig. 4), it will be found to have acquired the same 
power of attracting: iron filings, and throwing them into the magnetic 
curves. By a slight variation of the experiment (fig. 5) it may be shown 
that the contact of the magnet is not necessary to impart a temporary 
magnetic power to the iron. Here mm’ will be the neutral line. 
Steel possesses the same properties, in respect to the magnet, as iron, 
resisting its influence, however, more strenuously. By continuing the 
approximation for a long time, or by stroking the steel with the magnet, the 
former becomes permanently magnetic. Soft iron then differs from steel, 
in acquiring magnetism very readily, and losing it almost entirely when 
removed from the exciting cause; steel, on the other hand, is slow in 
acquiring magnetism, but retains it for a long time. The property by 
which a magnet developes magnetism in a piece of iron or steel is called 
magnetic induction. 
Hardened steel is used almost exclusively in the construction of artificial 
magnets, which are known from their shape, as magnetic needles, bar 
magnets, and horse-shoe magnets. A magnetic needle consists generally 
of a lozenge-shaped bar (fig. 6). with its centre, c, resting on a fine pivot. 
In the best needles this centre is of agate. The magnetic needle is 
sometimes suspended by a fine thread. 
Some magnets have three, or even more poles, as may be shown by 
laying a piece of paper on them, and sifting iron-filings upon the paper. 
The centres of attraction will indicate the various poles (fig. 7). 
All bodies, in their relations to magnetism, may be divided into three 
classes : magnetic, or those which are attracted by both poles; indifferent, 
those on which no action whatever is exerted by the magnet; and 
diamagnetic, those which are repelled by both poles. The existence of this 
latter property was first discovered by Faraday. The bodies exhibiting it 
are bismuth, antimony, zinc, tin, &c. Tron, nickel, and cobalt, are examples 
of magnetic bodies ; and ether, alcohol, and gold, of indifferent. 
2. Of the Magnetic Action of the Earth. 
A magnetic needle, suspended horizontally to a silk thread, or sustainea’ 
on a pivot, exhibits a tendency. if left to itself, to take up a definite position 
315° 
