162 



ON MAGNETISM. 



It will be remarked" that the part due to induced 

 magnetism diminishes much more rapidly than that 

 due to permanent magnetism. 



73. Method of measuring the amount of magnetism 

 produced in a magnet by terrestrial induction. 



It is assumed in the following process that the 

 law of distribution of induced magnetism, as regards 

 the length of the magnet, is sensibly the same as 

 that of permanent magnetism : and in fact the law 

 at which we have arrived theoretically of induced 

 magnetism, that the magnetism is exhibited as a coat- 

 ing at the end, is sensibly the same as that found 

 experimentally for permanent magnetism, Article 12. 

 The method adopted at the Kew Observatory for mea- 

 sure of the induced magnetism makes use of that part 

 induced by the terrestrial vertical force, which in this 

 magnetic latitude is large. As in Article 26, a deflexion- 

 apparatus is to be used, in which it is made certain, in all 

 stages of the operation, that the deflected needle (carry- 

 ing a reversed telescope) occupies the same position with 



