278 MAGNETISM 



branch H is + for both, while dB is + in one, in the other, and 



Tms'H 

 the total energy supplied is the difference - . It is easily 



seen, then, that in the complete cycle the energy supplied i> 



rudB = j_ x guin of all such t . as pQQ/p/ = area of 



J 4?r 4-7T 4-7T 



hysteresis loop. 



If we put B = H + 471-1, then dB = (III + 4WI, and 



Now 7j is round a cycle, since H returns to its initial 



value. Then the energy supplied is f lldl. Or if the h\Mt 

 curve is drawn for II and I, tin- energy supplied i> represented by 

 the area of the loop. Since tin- iron returns to its original con- 

 dition after a complete cycle, and mcanw bile there i>an addition of 

 energy, we may conclude thai thi^ en ( i and is tin- 



equivalent of ihc heat \\hirh appear-. 



It can be shown by a thennodvnaniie cycle that if u is eon-tant 

 for a given temperature so that the iron can be magnet i^-d l\ a 

 reversible process, and if yu varit-s with change of temperature, then 

 energy must be supplied to a magnet iM-d body to keep the 

 temperature constant if the magnrf i-at ion i> mcica-ed. The 

 amount thus supplied * is 



_ 



' 47T fl dtf 



We can onlv applv this formula to n-al iron in the initial 

 investigated by Lord Ha\ K -iglu l-'ig. ^10. and in the small revci 

 from subsequent stage > \\lure the \ ti>ation is practi- 



cally reversible. We cannot diducc the heat mjuired to keep tlie 

 temperature constant in a non -reversible process, nor the In at 

 appearing in a non-reversible cvclc. It is possible that if we could 

 account for the heat required in the reversible process by some 

 physical explanation, the same explanation would account for the 

 heat dissipated in hysteresis. But the mechanical illustration gi\en 

 (p. 279) would suggest that they are distinct, that the magnetisation 

 consists of two parts, one strictly reversible and with the heating 

 also reversible, another part non-reversible, dissipating energy by 

 something analogous to friction. 



In connection with this subject some very interesting experi- 



* Thomson, Applications of Dynamics to Physics and Chemistry, p. 103. 



