MAGNETISATION. 385 



CHAPTER V. 

 ON MAGNETS. 



406. MAGNETISATION. In order to magnetise a body endowed 

 with coercive force a steel bar, for instance it may be placed in a 

 constant magnetic field, or its various parts may be successively 

 submitted to the action of a variable field, like that produced by 

 rubbing it with a magnet. 



This latter method is the oldest, and is that most frequently 

 employed. Each point takes at every moment a magnetisation 

 depending on that already obtained, on the actual resultant force, 

 and, to some extent, on the time during which it acts. 



Whatever method may be employed, part of the magnetism 

 developed is temporary, and disappears with the external forces. 

 Another part is permanent or residual, and all experiments show that 

 these two kinds of magnetisation have a maximum limit. 



The temporary magnetisation is greater, and the residual magneti- 

 sation less, with iron than with steel; but both have a maximum 

 which depends only on the quality of the substance. In the case 

 of very feeble forces magnetisation seems to be altogether temporary 

 both for steel and for iron. 



The general problem of magnetisation would consist in determining 

 what would be the temporary magnetisation at each point of a body 

 of given shape, and nature, subjected to known forces ; and what, 

 when these forces are suppressed, would be the residual magneti- 

 sation. This problem has only been solved theoretically in a very 

 small number of cases. 



407. INDUCTION OF A MAGNET ON ITSELF. DEMAGNETISING 

 FORCE. The total magnetism of a magnet must be considered as 

 made up of two parts, the one due to magnetic masses kept fixed by 

 the coercive force, and which may be called rigid magnetism, the 

 other resulting from the induction of the first on the magnetic body, 

 and which constitutes induced magnetism. 



The internal action of induced magnetism is clearly in the opposite 



c c 



