302 CONSTITUTION OF MAGNETS. 



an infinitely small magnet, or like the system of two infinitely near, 

 equal, and contrary masses (151). The magnetic moment of the 

 magnet for unit volume is equal to hq. 



The value of the ratio h varies with different magnetic bodies, 

 and for the same body the value of q at each point depends on the 

 degree of magnetisation ; external actions increase or diminish with 

 the product hq. In bodies which have no coercive force, nothing 

 prevents the movement of fluids in the interior of a magnetic particle ; 

 equilibrium can only exist when the resultant of all the forces, 

 internal as well as external, is zero for every point of the molecule ; 

 on the contrary, in a body endowed with a certain coercive force, 

 which acts like friction, it is sufficient if this resultant be less than 

 the value given for the coercive force. 



Poisson's theory is not bound up with the hypothesis of two 

 fluids, but it is more difficult to free it from this particular concep- 

 tion of the structure of magnetic media. 



317. SIR W. THOMSON'S THEORY. We shall prefer to explain 

 the theory of magnetism in the form given to it by Sir W. Thomson. 

 This theory agrees with that of Poisson in its essential results, but 

 it has the advantage of being independent of the idea of fluid, and 

 of any hypothesis on the constitution of the medium, so that it seems 

 to be in closer agreement with experimental facts. The fundamental 

 notion is to consider any given portion of a magnet as being a 

 complete magnet, defined by the direction of the axis and by its 

 magnetic moment that is to say, as an infinitely small magnet 

 having masses + m and - m at its ends, a length <&, and therefore a 

 magnetic moment equal to mds. 



318. INTENSITY OF MAGNETISATION. That being admitted, the 

 term intensity of magnetisation I at a point, is the quotient of the 

 magnetic moment of a volume element by the volume itself in 

 other words, the value of the moment for unit of volume. 



We shall have thus 



mds 



This intensity of magnetisation I, represents the product hq in 

 Poisson's theory. 



The intensity of magnetisation is a geometrical magnitude defined, 

 like a force, by its direction, which is the magnetic axis of the 

 volume element, and by its numerical value ; it will therefore be 

 represented at every point by a straight line of given direction and 

 magnitude. 



