34 THE MAGNETIC CIRCUIT [ART. 15 



A piece of iron can also be carried through a hysteresis cycle 

 mechanically. Thus, instead of changing the excitation, the 

 sample may be moved to a weak field, reversed, and returned to 

 its original location. The relation between B and H, however, 

 will be the same in either case. 



An important feature of the hysteresis cycle is that it requires 

 a certain amount of energy to be supplied by the magnetizing 

 current, or by the mechanism which reverses the iron with 

 respect to the field. It is proved hi Art. 16 below that this energy 

 per cubic unit of iron is proportional to the area of the hysteresis 

 loop. This energy is converted into heat in the iron, and therefore 

 from the point of view of the electromagnetic circuit represents 

 a pure loss. If the cycles of magnetization are performed in 

 sufficiently rapid succession, for instance by using alternating 

 current in the exciting winding, the temperature of the iron rises 

 appreciably. 



The phenomenon of hysteresis is irreversible; that is to say, 

 it is impossible to make a piece of iron to undergo a cycle of mag- 

 netization in the direction opposite to that indicated by arrow- 

 heade, in Fig. 7. If it were reversible the loss of energy occasioned 

 by performing the cycle in one direction could be regained by 

 performing it in the opposite direction. In this respect the 

 hysteresis cycle differs materially from the theoretical reversible 

 cycles studied in thermodynamics, and reminds one of an irre- 

 versible thermodynamic cycle, in which friction or sudden expan- 

 sion is present. 



15. An Explanation of Saturation and Hysteresis in Iron. 

 While the physical nature of magnetism is at present unknown, 

 there is sufficient evidence that the magnetization of iron is 

 accompanied by some kind of molecular change. Let us assume, 

 in accordance with the modern electronic theory, that there is an 

 electric current circulating within each molecule of iron, due to the 

 orbital motion of one or more electrons within the molecule. Each 

 molecule represents, therefore, a minute electromagnet acted upon 

 by other molecular electromagnets. In the neutral state of a piece 

 of iron, the grouping of the molecules is such that the currents 

 are distributed in all possible planes, and the external magnetic 

 action is zero. Under the influence of an external magnetomotive 

 force the molecules are oriented in the same way that small mag- 

 netic needles are deflected by an external magnetic field. With 



