266 



Professor Fleming 



[June 5, 



elaborate explanation of it. It is sufficient to say that when iron is 

 magnetised and demagnetised, or carried round a cycle of magnetisa- 

 tion in which its direction of magnetisation is first in one direction 

 and then in the other, this process involves the expenditure of energy, 

 and such dissipation of energy is spoken of as the hysteresis loss in 

 iron. It would occupy too much time to attempt to explain in full 

 detail the manner in which this dissipated energy can be measured. 

 As a matter of fact, the method we adopted was the laborious but 

 exact one of delineating a complete magnetisation curve of the iron, 

 by means of observations taken with the ballistic galvanometer for 

 various maximum values of the magnetising force. In this way we 

 were able finally to arrive at a curve which represented by its ordi- 

 nates the value of the hysteresis loss in the iron in ergs per cubic 



2000 4000 6000 8000 10.000 I2.00P 



maximum induction during cycle. 



Fig. 19. 

 Yariation of hysteresis loss in soft iron with temperature. 



centimetre per cycle, and the abscissae the maximum value of the cor- 

 responding magnetic induction. AYhen curves had been drawn out 

 (see Fig. 19) from all the many hundreds of observations for the case 

 of the same soft iron ring at ordinary temperatures and at the tem- 

 perature of liquid air, we found little or no sensible difference 

 between them. The result is, then, that there is no appreciable 

 change in the magnetic hysteresis loss of very carefully annealed soft 

 Swedish iron when cooled to these low temperatures.* With regard 

 to the hard iron, although the permeability is increased, it is most 

 difficult to say yet whether the hysteresis is increased or not, as 

 every fresh reduction in temperature of the iron alters its physical 



* The iron used iu all these experiments was a sample of Sankey's trans- 

 former iron, kindly sent to us by Mr. R. Jenkins. 



