130 



Mr. B. L. Wills on the Effect of 



the ordinary temperature of the atmosphere. Reannealing 

 completely restores the iron to its original condition. 



Fig. 7 shows the variation of the hysteresis with time by 



Fig-. 7. 



fc 2000 











B=5000 

















s 







7~£t VPE&A TU/?£ 



/6S°c 



















T/ME /N DAYS. 



prolonged heating at 165° 0. for an induction of 5000. It 

 will be noticed that the hysteresis increases rapidly at first, 

 reaching a maximum value after about four days. Then it 

 decreases slightly to a constant value. 



Experiments on a Tungsten Alloy. 



The rate of change in the permeability of iron as the 

 temperature approaches very near to the critical temperature 

 is so great that, throughout a range of several degrees in 

 this region, only approximate values of the hysteresis could 

 be obtained. Moreover, at such high temperatures the iron 

 is in such a critical state that it was found that the process of 

 demagnetizing by reversals does not entirely wipe out all the 

 effects of previous magnetism, although the residual effect is 

 only noticeable in extremely weak fields. 



In the case of a specimen containing 4*5 per cent, tungsten 

 the passage from the magnetic to the non-magnetic condition 

 is much more gradual, and the demagnetizing process, even 

 at the highest temperatures, removed all trace of residual 

 magnetism. The variation of hysteresis loss with temperature 

 in the neighbourhood of the critical temperature can there- 

 fore be much more satisfactorily examined with this alloy 

 than with iron. 



The results obtained at different temperatures are given in 

 Table III., and hysteresis-induction curves plotted in fig. 8 

 (p. 132) . From these curves the hysteresis-temperature curves 

 shown in fig. 9 have been obtained ; the scale of ordinates given 

 applies to the curve for B = 6000, the curve for B = 2000 is 

 drawn to a scale five times as large. It will be seen that as 



