396 Professor J. A. Ewing [May 22, 



changes of magnetising force. This tendency to lag behind is called 

 magnetic hysteresis. 



We have a manifestation of hysteresis whenever a magnetic metal 

 has its magnetism changed in any manner through changes in the 

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

 confined to what I have called the first stage (a, Fig. 1). Residual 

 magnetism is only a particular case of hysteresis. 



Hysteresis comes in whatever be the character or cause of the 

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

 the molecules as make them become unstable. The unstable move- 

 ments are not reversible with respect to the agent which 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 when the mag- 

 netism 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 

 w^aste is proportional to the area of the loop in our diagrams. This 

 energy is dissipated ; that is to say, it is scattered and rendered 

 useless : it takes the form of heat. The iron core of a transformer, 

 for instance, which is having its magnetism reversed with every 

 pulsation of the alternating current, tends to become hot for this 

 very reason ; indeed, the loss of energy which happens in it, in conse- 

 quence of magnetic hysteresis, is a serious drawback to the efficiency 

 of alternating-current systems of distributing electricity. It is the 

 chief reason why they require much more coal to be burnt, for every 

 unit of electricity sold, than direct-current systems require. 



The molecular theory shows how this waste of energy occurs. 

 When the molecule becomes unstable and tumbles violently over, it 

 oscillates and sets its neighbours oscillating, until the oscillations 

 are damped out by the eddy currents of electricity which they 

 generate in the surrounding conducting mass. The useful work that 

 can be got from the molecule as it falls over is less than the work 

 that is done in replacing it during the return portion of the cycle. 

 This is a simple mechanical deduction from the fact that the move- 

 ment has unstable phases. 



I cannot attempt, in a single lecture, to do more than glance at 

 several places where the molecular theory seems to throw a flood of 

 light on obscure and complicated facts, as soon as we recognise that the 

 constraint of the molecules is due to their mutual action as magnets. 

 It has been known since the time of Gilbert that vibration greatly 

 facilitates the process of magnetic induction. Let a piece of iron be 

 briskly tapped while it lies in the magnetic field, and it is found 

 to take up a large addition to its induced magnetism. Indeed, if we 

 examine the successive stages of the process while the iron is kept 

 vibrating by being tapped, we find that the first stage (a) has practi- 



