378 



ELEMENTS OF ELECTRICAL ENGINEERING. 



and 3t when, after the iron has been magnetized up to the point 

 /, the value of $ is slowly reduced to zero. The curve of cB 



and dt for decreasing values of 3{ 

 does not coincide with the curve for 

 increasing values of C K Further- 

 more, the total area opa represents 

 the work done upon the iron in mag- 

 netizing it up to the point /, and 

 the area pp'a represents the work 

 which is regained from the iron 

 when the magnetizing field drops 

 slowly to zero. The ivork regained 



; is less than the work required to 



magnetize the iron. The work 

 which is lost is represented by the 

 shaded area in Fig. 16. 

 The scale to which area in Fig. 1 6 represents work, that is, the 

 number of ergs represented by each unit of area, may be deter- 

 mined as follows : 



Abscissas represent values of &C to scale, so that we may write 



#- (.) 



Ordinates represent values of cB to scale, so that we may write 



Fig. 16. 



or 



& = by 

 d$ = b-dy 



Substituting these values of <^and d$> in equation (14) we 



have 



abV 



W 



47T 



05) 



in which a is the number of units of $ represented by one unit of 

 abscissa, and b is the number of units of cB represented by one 

 unit of ordinate in Fig. 1 6. Now \x - dy is the area between 



