186 



C. G. KNOTT 



As in the case of nickel, so here, the magnetic lag is positive for 

 the smaller and negative for the higher twists. The change of sign 

 occurs, however, at much higher values of twist. In Table V. we see 

 the relations between the twist, the range of intensity, and the area of 

 the cyclic curve, clearly shown. Others are included than those given 

 in Table IV. 



Table V. 



Thus, exactly as in the case of nickel, the range very soon ap- 

 proximates to its limiting value, while the area, once the critical 

 twist is passed, seems to grow rapidly as the twist increases. 



An interesting feature of the iron curves at the high twists is 

 that the greatest and least intensities do not occur at the limiting 

 twists. In other words, the curves have a distinct S shape with true 

 maximum and minimum points. This feature appears first in the 

 experiment for the twist ± 135°, for which the full numbers are not 

 here fiven ; but it does not become distinct and undoubted at both 

 limits of twist until the twist of ± 270° is attained. Thereafter it is an 

 invariable feature. 



In seeking for an explanation of these phenomena of magnetic 

 la"\ we must bear in mind the essential difference between experiments 

 in which the mechanical strain is the cause of magnetic change and 

 those in which magnetising force is the cause. Take for example the 



