86 Profs. J. A. Fleming and J. Dewar. On the 



The results of these observations are given in Table II, and these 

 observations are set out in the curve marked soft annealed iron in 

 fiff. 2. 



Table II. Variation of the Magnetic Permeability of Soft Annealed 

 Swedish Iron with Temperature. 



Magnetising force = 177 C.G.S. 



Temperature measured in platinum degrees by standard thermo- 

 meter PI. 



Temperature. Permeability. 

 2835 



- 20 2815 



- 40 2770 



60 2727 



- 80 2675 



100 2622 

 -120 2560 

 -140 2497 

 -160 2438 

 -180 2381 

 200 2332 



The results show that as the temperature rises up from 185 C., or 

 200 on the platinum scale temperature, up to the ordinary tempera- 

 ture, the permeability of the soft iron for the particular magnetising 

 force selected increases perfectly uniformly, the curve of increasing 

 permeability with temperature being nearly a straight line. 



In the next place, we have examined the hysteresis of the same soft 

 iron ring at different temperatures and for different maximum induc- 

 tions. These observations were carried out by taking a complete 

 series of hysteresis curves with the ballistic galvanometer, gradually 

 increasing the inductions from zero to 12,000. After the complete 

 hysteresis curves were obtained, their areas were carefully integrated 

 with an Amsler planimeter, and the values reduced so as to express 

 the hysteresis loss in watts per Ib. per 100 cycles per second, and 

 these values plotted in terms of the maximum value of the magnetic 

 induction per square centimetre of the iron core corresponding to 

 each particular hysteresis loss. Nothing would be gained by giving 

 the full details of all the observations by which these hysteresis 

 curves were obtained. They were exceedingly numerous, and the 

 tedious nature of the ballistic observations made it a matter of pro- 

 longed observation to secure the whole series necessary, but the final 

 results are shown in Table III. The curve in fig. 3 represents the 

 increase of hysteresis loss with induction, and the observations which 



