INDUCED MAGNETISATION 



249 



H to ON and then reducing it to zero is (OPAMO-CTAMC) 



4"7T 



= OPATCO. The work done in increasing the negative value 

 of H to OD is -L OCDO, or the total work done is T - OPACDO. 



47T 4-7T 



If we take the iron through several hundred cycles of H between 

 values + ON and ON, then in the later cycles its cycle of 

 magnetisation is represented very nearly by a closed loop as in 

 Fig. 193, the hysteresis loop. In each cycle the work done on the 



Fio. 193. 



iron piece is represented by X area of loop, and in each cycle the 



energy represented by this work is dissipated and appears as heat 

 in the iron. 



We shall return to this subject in Chapter XXI. 



Calculation of induced magnetisation is only 

 practicable when the permeability is constant. When /x is 

 constant we may calculate the magnetisation induced in a given 

 field in certain cases, of which we have given easy examples in the 

 sphere and the ring. But when /UL varies, as it does with ferro- 

 magnetic bodies as soon as H rises above a small value, hysteresis 

 comes into play and the equations B = /xH and I = /cH may cease 

 to have meaning. For example, suppose that, as in Fig. 192, H is 

 taken to the value ON, and is then reduced again to zero, B and 

 I still have positive values, that of B being represented by OC, and 

 there is no meaning in saying that B = /xH at this stage. 



Diamagnetic bodies. If a material, instead of concentrating 

 the lines of force upon itself, widened them out that is, if, taking 

 the fluid illustration of p. 235, it were like a hydraulic system in 



