the Intrinsic Field of a Magnet. 413 



intrinsic field of some kind o£ very large magnitude ; and we 

 are led, therefore, to think of the intrinsic field as the 

 combination of two fields, one a true magnetic field and the 

 other a field of force, not magnetic, arising from molecular 

 forces. These two fields may be distinguished as the 

 magnetic intrinsic field and the molecular intrinsic field. 



The extreme ease with which an external magnetic field 

 can turn the magnetic molecules shows that the molecular . 

 intrinsic field does not exercise any constraint on the 

 orientations of the molecular magnets, the only rotational 

 constraint to which they are subject being the magnetic 

 intrinsic field '*. On the other hand, the molecular intrinsic 

 field controls displacements and translatory movements of the 

 molecules, while the magnetic intrinsic field is not primarily 

 concerned with movements of this kind. 



Thermal action produces to and fro displacements which 

 increase as the temperature rises, and at the critical 

 temperature the molecular intrinsic field becomes very 

 small, as witnessed by the fact that elastic forces then 

 very greatly diminish. But, although thermal action does 

 not directly produce rotational vibrations of the molecules, 

 it is a, property peculiar to a ferro-magnetic substance that 

 rotational vibrations can be communicated to the molecules 

 from translatory movements in virtue of the mutual magnetic 

 forces existing beticeen them. If, for example, we consider a 

 pair ol magnetic molecules with their axes in alignment, a to 

 and fro displacement of one, at light angles to this axial line, 

 would produce a forced vibration of the other around its 

 centre, and the magnetic moment of the pair would diminish. 

 Thus it is by the intermediate agency of mutual magnetic 

 forces that thermal action affects the intensity of magneti- 

 zation. As the to and fro displacements of the molecules 

 become larger and larger with rise of temperature, ultimately 

 losing their oscillatory character at the critical temperature, 

 so the amplitude of the rotational vibrations becomes propor- 

 tionally wider and wider, and ultimately rotations are 

 developed at or above the critical temperature, and, in 

 consequence, ferro-magnetic properties disappear. Thus, at 

 the critical temperature, both the molecular and the magnetic 

 intrinsic fields become negligible or very small, and the 

 concurrent loss of elastic and magnetic properties at this 

 point finds a ready explanation. 



Briefly, the view here taken of the structure and behaviour 



* It is not vital to the view here taken to decide whether the 

 magnetic axis of the molecule is fixed within it, or whether it is free to 

 be orientated within the molecule. 



