treated according to Van der Waals's Equation. 341 



former positive value, the negative magnetic intensity passes 

 its residual value at D', reaches a point of instability at C ; , 

 and again rapidly changes to a high positive value at F, 

 thus completing a hysteresis loop. 



The reduced field l u corresponding to the point C is the 

 same as the maximum reduced coercive force if the magnetic 

 intensity falls abruptly at this point through zero to its 

 stable negative value. 



7. By means of equations (5) and (6) the variation with 

 temperature of Z u , the reduced field for instability corre- 

 sponding to the point C, can be determined. This has been 

 done, and is shown in the inset to fig. 2, from which it is 

 seen that the reduced field falls rapidly at first, but at a 

 diminishing rate, as the temperature progresses until it 

 reaches zero, when the temperature is 27/32T c , and the 

 residual intensity is equal to ^I . 



The hysteresis loop thus shrinks in area with rise of 

 temperature. 



If we put Z« = H tt /H c in equation (5), and write 



H,= ia'V, (7) 



then 



H u =a'I W(l-2mJ, .... (8) 



which shows that the field for instability depends on the 

 maximum intrinsic field (a'I 2 J, and on m u which is a function 

 of the temperature. 



8. If the analogy between fluid and magnetic hysteresis 

 is once more recalled, we see that in fluids hysteresis is not 

 usually in evidence, and it can only be observed under 

 special conditions ; on the contrary, in magnetism it is 

 always prominent, and it is only under special conditions 

 that it can be suppressed. In both fluids and magnetism 

 freedom from disturbance is favourable to the exhibition of 

 hysteresis, and agitation of any kind reduces it and may even 

 annul it. For example, when a ferromagnetic body is sub- 

 jected to mechanical agitation or to an alternating current, 

 hysteresis is suppressed partly or completely. Under these 

 circumstances magnetism is acquired with very great facility 

 on the application of a very small field, and the susceptibility 

 is then enormously great. In fig. 2 the broken line drawn 

 nearly parallel and close to the vertical axis represents 

 theoretically an anhysteretic curve of magnetization, and 



