CHAMBERS'S INFORMATION FOR THE PEOPLE. 



is done by magnetising separately a number of 

 similar bar or horse-shoe shaped pieces of steel, 

 and binding them together with the like poles 

 lying the same way. Fig. i shews the mode of 

 arrangement of a compound bar-magnet. The 

 bars are bound by a brass screw or frame to a 

 piece of iron at each end, in which the several 

 poles have their magnetism con- 

 centrated. Fig. 2 represents a 

 compound horse-shoe magnet, 

 with its armature attached. This 

 armature or keeper must be used 

 with all magnets, to prevent the 

 loss and ultimate disappearance 

 of their magnetism. It is simply 

 a piece of soft iron placed across 

 the opposite poles of a horse- 

 shoe magnet, or of a pair of 

 bar-magnets. Any magnet, not 

 shielded by its armature, has 

 its polarity disturbed and even 

 destroyed by the action of the 

 earth, which is itself an enor- 



Fig. 2. 



mous magnet. The soft iron is made, by induction, 

 an exact counterpart of the magnet, and the power 

 of the permanent poles is in this way even in- 

 creased, for a much greater weight can be sus- 

 pended from the armature than the single poles 

 can sustain. 



Magnetisation. There are several processes by 

 which a bar of steel may be magnetised. The 

 simplest way is to pass one pole of a strong mag- 

 net from end to end of the bar several times, and 

 always in the same direction. In this case the 

 magnetism of the end first touched will be the 

 same as that of the rubbing pole. 



A somewhat better way is to rub the steel bar 

 from the middle to one end with one pole, say the 

 north, of the magnet, and then from the middle to 

 the other end with the south pole. Each half is to 

 be rubbed the same number of times, and the end 

 rubbed by the north pole becomes a south pole. 



Another, and a very accurate method is that 

 shewn in fig. 3, which is employed for magnetising 

 needles. It is called the method by Divided 

 Touch. The bar, ns, to be magnetised, is placed 



ends to the bend, or in the opposite way, and 



Fig- 3- 



on a piece of wood, W, with its ends resting on the 

 opposite poles of two strong bar-magnets, NS 

 and SN. Two rubbing magnets are placed, with 

 their opposite poles together, at the middle of ns, 

 and making a small angle with it. They are then 

 moved away from each other to the ends of ns, 

 and brought back in an arch to the middle again. 

 If this is done a few times, the bar is fully mag- 

 netised, the positions of the various poles being 

 indicated by the letters in the figure. 



For horse-shoe magnets, the usual method is to 

 place the inducing magnet upright on the magnet 

 to be formed, with an armature across its ends, 

 as drawn in fig. 4. It is then moved from the 



258 



Fig. 4. 



brought round again in an arch to the starting- 

 point. 



Other methods of magnetising steel, which do 

 not require the aid of another magnet, are fur- 

 nished by the magnetic power of the earth, and 

 by the galvanic current But these will be 

 explained afterwards. 



Magnets, when freshly magnetised, have often 

 more magnetism than they can retain. In that 

 case, they gradually fall off in strength until they 

 reach a stationary point, when they are said to be 

 saturated. The strength of magnetism that can 

 be imparted permanently to any magnet, or its 

 saturation point, depends on the quality and 

 temper of its steel. If a magnet has got weakened, 

 its strength may be gradually restored by hanging 

 weights to its armature, and increasing them 

 gradually from time to time. On the other hand, 

 by repeatedly detaching the keeper abruptly, we 

 weaken the power of a magnet, and in this way it 

 is very easy to reduce one above saturation to its 

 fixed strength. It may be remarked generally, 

 that whatever disturbs the molecular condition, or 

 internal state of the particles of a magnet, affects 

 its power. A red heat will deprive it of all traces 

 of magnetism, and a blow, or a number of blows, 

 may do the same, or may even change the poles. 



Theory of Magnetism. The best known theory 

 of magnetism supposes that there are two mag- 

 netic fluids existing in the substance of the magnet, 

 and adhering to it. Each kind of fluid repels itself, 

 but attracts the other kind. In an unmagnetised 

 body, the two fluids are supposed to be mixed up 

 together, and so to counteract each other's effects. 

 Magnetisation consists, according to this view, in 

 separating the two fluids to opposite sides of the 

 body, or to opposite sides of each particle of the 

 body. 



Another more recent and more plausible theory 

 regards a magnet as an assemblage of minute 

 permanently magnetic particles, with their similar 

 poles all lying one way. We have seen that, 

 though a magnetic bar be broken into fragments, 

 each piece is a perfect magnet. This fact is sup- 

 posed to be a strong argument in favour of the 

 latter theory. To magnetise a body is to throw 

 its particles into a state of regular polarity, its 

 previous want of magnetism being due to the want 

 of this regularity. 



Ampere's theory, which connects the phenomena 

 of magnetism and of electricity, is another and 

 still more novel theory, which will be explained 

 under Electro-magnetism. 



Diamagnetism. It had been known since the 

 beginning of this century that the metals nickel 

 and cobalt were feebly magnetic, and could be 





