338 Dr. J. R. Ash worth on Magnetic Hysteresis 



The equation is a cubic in I, the intensity, and, therefore, 

 the rapid augmentation of intensity in the second stage may 

 be regarded as continuous along a curved path with a double 

 inflexion in it. It is this double inflexion of the isothermal, 

 having two stable parts joined by an unstable portion, which 

 gives rise to magnetic hysteresis. 



4. According to these views, we should expect to find not 

 only a critical temperature (T c ) but also a critical intensity 

 (I c ) and a critical field (H c ), expressions for which are easily 

 obtained from the ferromagnetic equation. 



There is ample evidence that a critical temperature (or 

 temperature-interval) exists for magnetism, and the evidence 

 for a critical intensity is clear if we regard this intensity as 

 the point where ferromagnetic qualities change to para- 

 magnetic qualities at the critical temperature — in other 

 words, where the critical isodynamic changes its curvature. 

 The evidence for a critical field is, however, not quite so 

 obvious ; but, as Curie pointed out *, there is at low fields an 

 abrupt change of direction of the isothermal just below the 

 critical temperature from being nearly vertical to being 

 nearly horizontal, whilst just above the critical temperature 

 the isothermals are straight lines inclined at a small angle 

 to the axis of H. In the angular space, included between 

 the horizontal and inclined isothermals, there must be some 

 field for which the abruptly changing isothermal passes to 

 ihe smooth linear isothermal, and this may be treated as the 

 critical field. 



5, The specific properties of any ferromagnetic body 

 -depend on the values assigned to the constants a\ I , and 

 R/ ; but, by eliminating these quantities, a reduced equation 

 can be obtained which gives the relations of I, H, and T in 

 their most general form. Such an equation can be written 

 by treating I, H, and T as functions of the critical constants 

 or of some other well-defined values. Thus let 



7 H X A T 



Z= H C ' m= V andn =T? 



where I is the maximum value of the intensity and is equal 

 to 3I C ; then the reduced equation is 



(l + 8V)'(i-l)-8a 



(2) 



We can trace the reduced curve of I = </>(H) from this 

 * Curie, (Euvres, p. 309. 



