

— dA — r (.c\ rf95') - np, (iö) - (rf^, .fp), 



o u 



trom which easily follows: 



Integrated with respect to the wiiole Held tliis liecomes: 



dA = — ids rb . (/ .!>. (/« (19) 



We can always split up the vector -'J into two parts, .'V, for 

 which holds div ^' = 0, and -p', for which holds curl .D'n=.0'). 

 Taking into consideration that generally 



on integration over the whole space, when 



div vl = 0, curl '^ =: 0, 

 we get : 



Ö 



'^''=raM^-'-^' 



Making use of the equation : 



iB = .p + '53^, 

 we get : 



d4 = — CdS p;rf.p° + CdS Cmd.^' . da. 



As in the first term we can .again split up -^ into -p" and .^', in 

 which -O" is independent of « — .p° being detern)ined by the current 

 J — and as tiie product ■•)'(/ p" integrated over the whole field 

 yields zero, this term will vanish, so that there remains; 



dA = ids I -- rf.fp» .da (20) 



In this denotes the change of the magnetisation in consequence 



da 



of a change da, in which tlie external electromotive forces and also 



the coefilcients determining the conductivity, remain unchanged. 



') hi general we shall understand by f;" tiie intensity of the field as it would 

 be witliout the presence of the irou, ii representing the real strength of the field. 

 The difference is ■,'. 



