CHAPTER XII. 



INDUCED MAGNETISM. 

 PHYSICAL PHENOMENA. 



458. REFERENCE has already been made to the well-known fact that a 

 magnet will attract small pieces of iron or steel which are not themselves 

 magnets. Here we have a phenomenon which at first sight does not seem 

 to be explained by the law of the attractions and repulsions of magnetic 

 poles. It is found, however, that the phenomenon is due to a magnetic 

 " induction " of a kind almost exactly similar to the electrostatic induction 

 already discussed. It can be shewn that a piece of iron or steel, placed in 

 the presence of a magnet, will itself become magnetised. Temporarily, this 

 piece of iron or steel will be possessed of magnetic poles of its own, and the 

 system of attractions and repulsions between these and the poles of the 

 original permanent magnet will account for the forces which are observed to 

 act on the metal. 



It has, however, been seen that pairs of corresponding positive and 

 negative poles cannot be separated by more than molecular distances, so 

 that we are led to suppose that each particle of the body in which magnetism 

 is induced must become magnetised, the adjacent poles neutralising one 

 another as in a permanent magnet. 



Taking this view, it will be seen that the attraction of a magnet for an 

 unmagnetised body is analogous to the attraction of an electrified body for a 

 piece of dielectric ( 197), rather than to its attraction for an uncharged 

 conductor. The attraction of a charged body for a fragment of a dielectric 

 has been seen to depend upon a molecular phenomenon taking place in the 

 dielectric. Each molecule becomes itself electrified on its opposite faces, 

 with charges of opposite sign, these charges being equal and opposite so that 

 the total charge on any molecule is nil. In the same way, when magnetism 

 is induced in any substance, each molecule of the substance must be supposed 

 to become a magnetic particle, the total charge of magnetism on each particle 

 being nil. It follows that the attraction of a magnet for a non-magnetic 

 body is merely the aggregate of the attractive forces acting on the different 

 individual particles of the body. 



459. Confirmation of this view is found in the fact that the intensity of 

 the attraction exerted by a magnet on a non-magnetised body depends on 



