298 CONSTITUTION OF MAGNETS. 



CHAPTER II. 

 CONSTITUTION OF MAGNETS. 



310. MAGNETIC FILAMENTS. The experiment of breaking a 

 magnetised bar demonstrates this central fact that any volume 

 element of a magnet is itself a complete magnet, having in its then 

 state a magnetic axis and a definite moment. We say in its then 

 state, for it is clear that if the volume element instead of being 

 conceived as separated from the surrounding medium, were really 

 detached, it would no longer retain the same state as when it formed 

 part of the general mass. Let us consider two molecules placed 

 end to end, and only touching with their opposite poles ; if they 

 are equally magnetised, the action for any external point would 

 reduce itself to that of its two free ends. In like manner, if a 

 series of molecules equally magnetised are placed end to end, all 

 the magnetic axes being arranged on the same line, the external 

 action of the linear magnet thus constructed still reduces to that 

 of its two ends, each intermediate point giving rise to equal and 

 contrary actions which neutralise each other. Such a system of 

 magnetised particles constitutes a uniform magnetic filament. 



311." FREE MAGNETISM. But if the magnetisation in this line 

 of molecules is variable, at each point there will be a certain quantity 

 of apparent or free magnetism, equal to the differences of the mag- 

 netic masses of two adjacent molecules in contact. If we suppose, 

 for instance, that the magnetisation diminishes from the middle of 

 the filament to the end, we see that on one of the halves of the 

 linear magnet, there will be an excess of positive magnetism distri- 

 buted according to a certain law, and on the other half an equal 

 excess of negative magnetism. The magnetic filament thus con- 

 structed is no longer uniform, but it is evident that we can regard it 

 as the resultant of the juxtaposition of uniform filaments of different 

 lengths. 



