EFFECT OF TWIST OX THE MAGNETIZATION" OF NICKEL AND IRON". 199 



magnetization curve is opposite to what holds in weak fields for the 

 same angle of twist, as will be seen from Fig. 17. These experiments 

 sufficiently prove that the reversal of hysteresis not only depends 

 on the amount of twist, hut also on the strength of the magnetizing 

 field, the phenomenon appearing in lower and lower magnetizing 

 fields as the twist is taken greater and greater. 



The above statement is confirmed by further experiments made 

 with 1 mm. wire, the range of twist amounting to ± 'I e . The curve 

 (Fig. 18) for £=0.34 has the course usual for small twists, so that 

 there is true lagging on untwisting. But when the field strength is 

 increased to 7.3 units (see Fig. 19), the going curve lies above the 

 returning curve, and thus the hysteresis turns out to be negative. 

 Thus for this amount of twist, the magnetizing force was sufficient to 

 cause the reversal. 



Form of the eurer when the reversal is about to take place. — One im- 

 portant point still remains to be investigated. We must examine 

 what form the curve takes just as the reversal of hysteresis is about 

 to occur. 1 had the satisfaction of obtaining the desired curve by 

 subjecting the wire to a twist of ± 2°.o per cm. under the action of 

 the vertical component of the terrestrial magnetic force. The curve 

 just on point of reversal (see Fig. '20) is no more double looped, but 

 becomes more complex. Examining the changes of magnetization -, 

 we notice that on twisting, there is always increase of magnetization 

 till the extreme limit of twist is reached, while on untwisting, the 

 curve, instead of going below the former path, goes above it for a 

 certain distance but before returning to the line of zero twisting, cros- 

 ses the course taken in going, and finally passes below it. Thus the 

 curve is four looped. The appearance of four loo;» marks the transi- 

 tion in the nature of hysteresis, and any further increase of the mag- 

 netizing field produces reversal of hysteresis, the curve becoming 



