248 



MAGNETISM 



It is to be noted that the energy supplied only gives us the 

 energy stored, when the process of magnetisation is " reversible," 

 so that a given field produces a given induction, as it does when 

 ju, is constant. When the magnetisation may be the same for quite 

 different fields the energy spent is different for different paths 

 from one condition to another, and our investigation does not 

 lead to a value for the energy stored in the iron. But we may 



find an expression for the energy 

 supplied in any change. 



The energy supplied during 

 a cycle when the permeability 

 varies. If /u. varies we can represent 

 the energy supplied by an area on the 

 diagram, Fig. 191, which gives the 

 relation between H and B. Let the 

 curve OPQA represent the relation 

 between B and H when H is increased 

 from ZQTO to ON in a piece of iron 

 previously unmagnetised. 

 FIG. 191. Let PQ represent a small step in 



the process; then if we deal \\ith 

 1 c.c., the work done in the step PQ is HdB/4?r = PR X RS/4ir 



= 4- X area of strip PS. The total work done as H changes from 



to ON is j- x the sum of such strips, or ^- X area of OPAMO. 



If when H has reached the value ON it is diminished again, \M- 

 know that B does not fall off as would be represented by the curve 

 AQPO, but somewhat as represented by the curve AT( I) in rig. 192. 



M 



FIG. 192. 



When H is reduced to zero, induction OC still remains in the iron 

 and it is necessary to reverse H and give it a value, such as OD, 

 before the induction is destroyed. The work done in increasing 



