MAGNETISM AND ELECTRICITY. 



MAGNETISM. 



A NCIENTLY, there was discovered in Mag- 

 2~\ nesia, in Asia Minor, a kind of iron ore 

 which had the remarkable property of attracting 

 other kinds of iron or steel. This ore afterwards 

 received the name of loadstone, but from Magnesia 

 we derive the terms magnet and magnetism. 



Not only does the loadstone possess this power 

 of attracting iron itself, but it can communicate its 

 virtue to steel A bar of well-tempered steel, if 

 repeatedly rubbed by it, acquires exactly similar 

 properties, and is termed an artificial magnet, 

 while the other is called a natural magnet. In 

 point of strength and convenience, artificial mag- 

 nets are far superior to natural ones, and are 

 therefore more commonly used. 



If we take a bar of steel, which has been thus 

 rubbed by a loadstone, and to which the name 

 of bar-magnet is usually given, and scatter iron 

 filings over it, they will adhere in tufts at both 

 ends, but none will be found at the middle of the 

 bar. Or we may put the magnet under a sheet of 

 pasteboard, and sprinkle the filings over the sheet. 

 We shall then find the small particles of iron 

 arrange themselves in beautiful curved lines round 

 both ends of the magnet. It appears from this 

 that the chief power of the bar lies at the two 

 ends, round which, as centres, the curves are 

 formed. 



These centres of magnetic force are called the 

 poles of the loadstone or magnet, from another 

 peculiar property which they are found to possess. 

 If a magnetic bar be suspended by a thread, or be 

 delicately poised on a pivot, it will not rest till it 

 has settled in one position, which is in a direction 

 nearly north and south. If moved from this posi- 

 tion, it returns to it again, the same end always 

 turning towards the north. That end which so 

 points to the north is called the north pole of the 

 magnet, and the other its south pole. 



We can never have a magnet with only one 

 pole. The two are always found together, and are 

 opposite both in their properties and position. If 

 we break a magnet in two, we do not have two 

 pieces, one all north pole, and the other all south ; 

 but we have two complete magnets, and this how- 

 ever many times we break the bar. To this twin 

 exhibition of force residing in opposite sides of a 

 body is given the name of polarity. We shall find 

 that it is not peculiar to magnetism, but is a 

 feature of all electric phenomena. Sometimes, 

 indeed, we have in the same bar more than one 

 pair of poles ; but this is owing to some irregu- 

 larity of the steel temper, or of the mode in which 

 the magnetism has been communicated. When 

 more than one pair of poles occurs in a magnet, 

 they are called consecutive points. They render a 

 magnet practically worthless. 



The action of the poles of one magnet on those 



of another is remarkable. If we bring two north 



poles together, they repel each other ; or, if we 



bring two south poles together, they do the same. 



17 



But if we bring a north near to a south pole, they 

 attract each other powerfully, and cling together. 

 From this simple experiment we deduce the two 

 general laws of magnetic action : Like poles repel 

 each other : unlike, attract each other. It is thus 

 very easy to tell the north and south pole of any 

 magnet. We have simply to try its action on a 

 needle suspended so as to point north and south. 



Magnetic Induction. A bar of soft iron has of 

 itself no power to attract iron filings, or another 

 piece of iron. In presence of a magnet, however, 

 it instantly assumes this power. When we sus- 

 pend from, say, the south pole of a magnet a short 

 rod of iron, its free end attracts filings, just as the 

 magnet would do, and the iron has become, in 

 every way, like a magnet It has two poles, that 

 end being a north pole which is next the south 

 pole of the magnet. The same thing is seen, 

 though in a less powerful degree when the magnet 

 is simply brought near to the soft iron. But the 

 magnetisation of the iron, though perfect, is merely 

 temporary. The moment we take away the mag- 

 net, its magic power is gone, and the filings drop 

 off. 



This action of a magnet on soft iron, communi- 

 cating to it a temporary magnetic polarity, is 

 termed induction. All bodies, as we shall see 

 under diamagnetism, are magnetic that is, influ- 

 enced by magnetism in one way or another. But 

 induction proper, by which two magnetic poles are 

 excited, is confined to iron and its varieties. Even 

 here there is a remarkable contrast The influence 

 of a magnet on tempered steel is very different 

 from that on soft iron. Steel will not assume the 

 polar state at once, but only after repeated friction 

 with a magnet. Yet, once magnetised, it retains 

 its polarity with equal stubbornness. The cause 

 of this is not well understood ; but it is usually 

 ascribed to the presence with the particles of steel 

 of a coercitive force, as it is called, which resists 

 equally the communication and the abstraction of 

 the magnetic state. 



Induction, or the action of a polarised body 

 inducing a similar polarised state of another body 

 in its vicinity, is not confined to magnetism, but 

 appears to be manifested in every case of polarity. 

 We shall see, under Electricity, a similar influ- 

 ence possessed by a body electrically excited or 

 polarised. 



Forms of Magnets. In place of the bar shape, 

 it is very often more convenient to give the mag- 

 net the form of a horse-shoe, so that both poles 

 may be brought to act on the same object When 



Fig. I. 



powerful magnetism is desired, it is better not to 

 increase the mass of the magnet, but to form a 

 magnetic magazine, or compound magnet. This 



