262 THE MOLECULAR PROCESS IN MAGNETIC INDUCTION. 



till a stage is readied wlien instability begins, and then reversal occurs 

 witli a rush. We thus find a close imitation of all the features tlui^ 

 are actually observed when iron or any of theotlier magnetic metals is 

 carried through a cyclic magnetizing process (Fig 12). The effect of 

 any such process is to form a loop in the curve which expresses the re- 

 lation of the magnetism to the magnetizing force. The changes of 

 magnetism always lag behind the changes of magnetizing force. This 

 tendency to lag behind is called magnetic hysteresis. 



A 



B 



Maanetic Force 



B 



A 



Fig. 12. — Cyclic reversal of magnetization in soft iron (AA), and in the 

 same iron when hardened by stretching (bb). 



We have a manifestation of hysteresis whenever a magnetic metal 

 has its magnetism clianged in any manner througli changes in the 

 magnetizing force, unless indeed the changes are so minute as to be 

 confined to what I have called the first stage («, Fig. 1). Eesidual 

 magnetism is only a particular case of hi/steresis. 



Hysteresis comes in whatever be the character or cause of the mag- 

 netic change, provided it involves such deflections on the part of the 

 molecules as make them become unstable. The unstable movements 

 are not reversible witli respect to the agent whicli produces them; that 

 is to say, they are not simply undone step by step as the agent is 

 removed. 



We know, on quite independent grounds, that wlien the magnetism 

 of a piece of iron or steel is reversed, or indeed cyclically altered in any 

 way, some work is spent in performing the operation — energy is being 

 given to the iron at one stage, and is being recovered from it at another; 

 but when the cycle is taken as a whole there is a net loss, or rather a 

 waste of energy. It may be shown that this waste is proportional to 



