228 



Professor J. A. Ewing 



[May 22, 



lagging was shown to be static in character, for it was in no way 

 dependent on the rate at which the process of loading and unloading 

 was performed. Other cases of static hysteresis in the thermoelectric 

 and mechanical qualities of iron were mentioned. 



Practically the most important instance was the hysteresis which 

 was observed when a piece of iron had its magnetism changed by 

 changing the magnetising force. When a piece of iron was first 

 magnetised, the magnetism B was developed by gradual increase of 

 the magnetising force H, in the way shown in Fig. 1. If at any 

 stage in the process, such as a, the magnetising force was made to 

 stop increasing, was reduced to zero, and was then reapplied in the 

 opposite direction, the magnetism changed in the way shown by the 

 curve acd of Fig. 2. And finally, if the magnetising force were 

 again reversed, so as to recover the direction and value it had at a, 

 the process followed was represented by the curve dea oi Fig. 3. 



I*,000 

 IZOOO 



10.000 



6.000 

 6.000 

 4.000 

 2000 

 



Fig. 1. 



Fig. 2. 



Fig. 3. 



The closed curve of Fig. 3 showed how the changes of magnetism 

 in this complete cycle of double reversal tended to lag behind the 

 changes of magnetising force. In consequence of this hysteresis, 

 energy was consumed in reversing the magnetism of iron, and it 

 could be proved that the energy consumed in each double reversal 

 was proportional to the area enclosed between the curves acd and 

 dea. 



This was the process that went on in the iron cores of trans- 

 formers when used for electric lighting in the alternate-current 

 system of supply. The magnetic cycle was gone through something 

 like 100 times a second, and as a rule the transformer was in circuit 

 continuously by day and night. Whether it was supplying lamps 

 and doing useful service, or whether it was not, the waste of power 

 due to hysteresis went on. It formed a very serious item in the cost 

 of alternate-current supply, for the effect was that a large part of 



