MAGNETIC EFFECTS OF TWIST. 187 



well-known case, so frequently discussed, of a wire subjected to a 

 continuously varying longitudinal field. Here in virtue of magnetic 

 retentiveness, there is always true magnetic lag as the field is dimi- 

 nished from its highest value. The magnetic force is, in part, simply 

 removed. But when the wire is twisted through a large angle, the 

 effect of untwisting the wire is no mere removal of twist, but is really 

 a superposition of an opposite twist. Now, we know that the induced 

 magnetism due to a given field is apparently destroyed by a reversed 

 field of smaller value. It is this effect, rather than the effect of mere 

 diminution of the field to zero, that is to be compared to the effect of 

 untwisting, especially if it is untwisting from a large twist. If the 

 twist is small, however, that is, not greatly beyond the limits of tor- 

 sional elasticity, the untwisting will be aided by the elasticity of the 

 wire, so that it will have something of the character of a mere undo- 

 ing. From this point of view, then, small twistings and untwistings 

 will be to a certain extent comparable to applying and removing mag- 

 netising force : while large twistings and untwistings are to be com- 

 pared rather to applying first a given magnetising force and then a 

 small reversed magnetising force. Hence for small cyclic twistings, 

 the magnetic lag is a true lagging effect ; but for large cyclic twistings 

 it becomes really a "priming" effect. The fact that the limits of tor- 

 sional elasticity for iron are much greater than the same for nickel 

 fits in admirably with the result established above that the critical 

 twist at which the lag changes sign is much higher for iron than it 

 is for nickel 



There is, however, another and perhaps a simpler explanation of 

 the phenomenon. It is suggested by some results of experiments of 

 the same nature which I have carried out very recently. The experi- 

 ments are not quite completed ; but enough has been done to show 

 that, in the case of nickel (and in certain circumstances, iron also) 



