128 



Mr, R. L. Wills on the Effect of 



produced, pass through the same point &=— 100 on the 

 vertical axis. Within this range of induction, therefore, we 

 have W = /(H)B — b. As near as can be determined 



W = 0-182rrtB-100. 



For values of B below about 1800 the values of W are 

 larger than those given by the equation. As the induction 

 is increased beyond 5000 the hysteresis rises less rapidly to a 

 maximum value and then decreases. This is in complete 

 accordance with the experiments of Searle and Bedford, in 

 \\ Inch the induction is varied by straining. In their experi- 

 ments, however, it was impracticable to continue the curve 

 much beyond the point of maximum hysteresis. When the 

 induction is varied by heating the curve may be traced until 

 the critical temperature is reached. For low fields we thus 

 get a closed curve, as shown in fig. 5, Curve I. 



From the following table, which is obtained from Table II. 

 by drawing curves showing the variation in hysteresis with 

 temperature for constant fields, we see that the smaller the 

 magnetic force the higher the temperature at which the point 

 of maximum hvsteresis occurs. 



H. i Temperature for 

 , maximum hysteresis. 



i H. Temperature for 

 maximum hysteresis. 



; 034 



0-68 

 102 



705 3 C. 

 610 „ 

 565 „ 



1-36 

 1-70 

 1-90 



480° C. 

 355 „ 

 290 „ 



Experiments on the relation between temperature and 

 magnetizing force in the magnetization of iron have been 

 described in a previous paper*, where a curve is given 

 showing the magnetizing force corresponding to the maximum 

 permeability for different temperatures. A curve very similar 

 to this is obtained by plotting magnetic force as ordinate, 

 and the temperature at which the maximum hysteresis is 

 reached as abscissa, the temperature giving the maximum 

 hysteresis for any magnetic force being, as near as can be 

 determined, the same as that at which the maximum per- 

 meability occurs for the corresponding magnetizing force. 



"Ageing" of Iron. 



The " ageing " of iron by contiuued heating has been 

 -already referred to. Mordeyf has examined the effect for 



* R. L. Wills, Phil. Mag. vol. 1. pp. 1 L-14, July 1900. 

 t Troc. Ptoy. Soc. vol. lvii. pp. 224-244, Jan. 1895. 



