ELECTROMAGNETS. MAGNETISM OF IRON. 



99 



The magnetization of a given portion of the iron core of a 

 transformer is reversed with each reversal of the alternating 

 current which flows through the primary coil, the iron is thus 

 repeatedly carried from a certain degree of magnetization in one 

 direction (a certain positive value of cB) to the same degree of 

 magnetization in the opposite direction (the same negative value 

 of cB) and back to the original degree of magnetization. Such 

 a magnetic double-reversal is called a magnetic cycle. At the 

 end of a cycle the iron comes to precisely the same condition that 

 it had at the beginning of the cycle. The importance of the 

 magnetic cycle in the determination of the amount of heat 

 produced by magnetization will be appreciated from the following 

 two statements : 



(a) When a mass of iron is magnetized along the cB and &C 

 curve op of Fig. 67 and then partially demagnetized along the 

 curve pp f , a portion of the work 



done upon the iron during the 

 first stage op is regained dur- 

 ing the stage pp', a portion is 

 lost as heat, and a portion re- 

 mains in the iron as energy of 

 magnetization; and no experi- 

 mental method has been devised 

 for determining the second and 

 third portions of work separ- 

 ately. 



(b) When, however, a mass of 

 iron is carried through a mag- 

 netic cycle, the algebraic sum 

 of the work spent upon the iron 

 during the cycle is lost as heat, 



inasmuch as the magnetic energy Fig 68 



in the iron is exactly the same 



at the beginning and at the end of the cycle. 



Figure 68 shows the relation between eB and $ during a 



