626 CONSTANTS OF MAGNETISATION. 



bar may be neglected, the intensity of the magnetisation of the bar 

 being I a , the value of the flow of induction across a closed circuit S, 

 which surrounds the central section, is 



Determining this flow by the induced discharge, and then re- 

 peating the same experiment with the coil alone, which will give 

 Q 2 = GIS, we get, by difference, 



If the bar forms a ring, and is surrounded by a ring-shaped coil 

 (754), the internal field is sensibly uniform when the section of the 

 bar is small in comparison with the diameter of the ring ; experiment 

 still gives in all cases the mean magnetisation. 



With rings there are only variations of magnetisation, and the 

 experiment says nothing as to the residual magnetism of the system , 

 and therefore on its total magnetisation. 



1200. INDUCED MAGNETISATION. The quantities which we 

 should endeavour to determine experimentally are, the coefficient 

 of magnetisation k, which Sir W. Thompson has called its magnetic 

 susceptibility (383), and the coefficient of induction /x = i + 4^, or 

 the magnetic permeability. 



These qualities vary with the magnetising force, and the problem 

 can only be completely solved in particular cases. 



The magnetising force results from the primitive field and from 

 the induced magnetisation (385), and consequently it varies from 

 one point to another. As the real state of a magnet cannot be 

 deduced from its external actions, we must confine ourselves to 

 the case in which, the field being uniform, the induced magnetism 

 itself gives either a uniform field, or one which may be neglected. 



In a uniform field of intensity $, if the field of the induced 

 magnetism is also uniform, the magnetisation I a of the body is con- 

 nected generally with the intensity of the field by an expression of 

 the form 



The value of the constant C is equal to -TT for a sphere, or for 



o 



an ellipsoid, to one of the coefficients - L, - M, - N the corre- 

 sponding axis of which is parallel to the field (388). We may add 



