MAXWELL'S THEORY. 403 



The magnetisation of iron, as a function of the field, must 

 therefore be represented by a curve such as OBA (Fig. 90), having 

 a point of inflexion; this first part of the curve has often been 

 confounded with the tangent passing through the origin, and 

 which gives the maximum value for k. Weber's theory does not 

 account for this variation of the coefficient of magnetisation for 

 small forces; nor, on the other hand, does it throw any light on 

 the nature of residual magnetisation. 



430. MAXWELL'S THEORY. In order to complete this last 

 link, while still adhering to the general theory, Maxwell supposed 

 that the medium had a kind of imperfect elasticity. He assumes 

 that the axis of the magnetic molecules revert to their original 

 position, after the suppression of the magnetising force, so long as 

 the rotation which they experience is below a certain value, but 

 that their axes retain a permanent deviation ft - /3 Q , when the 

 rotation /3 has been greater than the limiting value /3 . This 

 deviation ft - /3 Q characterises the permanent condition of the 

 molecule. 



This hypothesis undoubtedly dbes not represent the exact state 

 of the phenomena, but it may furnish an approximate idea, and 

 enable us to submit the problem to calculation. 



According to Maxwell, we may deduce the temporary magneti- 

 sation I and the permanent magnetisation I' by a calculation 

 analogous to the preceding. Putting 



we get thus : 

 When X<L, 



When X = L, 

 For L < X < D, 



D D 2 



