SECT, xxx.] INDUCTION OF MAGNETISM. 349 



magnets is characterised by a repulsive as well as an attract- 

 ive force, for a north pole repels a north pole, and a south 

 repels a south pole. But a north and a south pole mutually 

 attract one another, which proves that there are two distinct 

 kinds of magnetic forces, directly opposite in their effects, 

 though similar in their mode of action. 



Induction is the power which a magnet possesses of excit- 

 ing temporary or permanent magnetism in such bodies in its 

 vicinity as are capable of receiving it. By this property the 

 mere approach of a magnet renders iron or steel magnetic, 

 the more powerfully the less the distance. When the north 

 pole of a magnet is brought near to, and in the line with, an 

 unmagnetised iron bar, the bar acquires all the properties of 

 a perfect magnet ; the end next the north pole of the magnet 

 becomes a south pole, while the remote end becomes a north 

 pole. Exactly the reverse takes place when the south pole 

 is presented to the bar ; so that each pole of a magnet induces 

 the opposite polarity in the adjacent end of the bar, and the 

 same polarity in the remote extremity; consequently the 

 nearest extremity of the bar is attracted, and the farther 

 repelled ; but, as the action is greater on the adjacent than on 

 the distant part, the resulting force is that of attraction. By 

 induction, the iron bar not only acquires polarity, but the 

 power of inducing magnetism in a third body ; and, although 

 all these properties vanish from the iron as soon as the 

 magnet is removed, a lasting increase of intensity is generally 

 imparted to the magnet itself by the reaction of the 

 temporary magnetism of the iron. Iron acquires magnetism 

 more rapidly than steel, yet it loses it as quickly on the 

 removal of the magnet, whereas the steel is impressed with 

 a lasting polarity. 



A certain time is requisite for the induction of magnetism, 

 and it may be accelerated by anything that excites a vibra- 

 tory motion in the particles of the steel, such as the smart 

 stroke of a hammer, or heat succeeded by sudden cold. A 

 steel bar may be converted into a magnet by the transmis- 



