MAGNETISM AND TWIST IN IRON AND NICKEL. 499 



circumstances, when the magnetising force has been reduced to zero, the wire is left 

 magnetised positively. If, however, we had first applied the force — b, and then reduced 

 it to zero by the superposition of the force + b, the wire would have been left negatively 

 magnetised. Thus the ultimate magnetic effect of the successive application of two equal 

 and opposite forces, the final result being the force zero, depends entirely upon the 

 order in which the balancing forces are applied. Such being the case for forces acting 

 in the same line but in opposite directions, it is not surprising that something similar 

 results when the superposed forces are mutually orthogonal. 



It is evident, in fact, that a combination of circular and longitudinal magnetic forces 

 impresses upon the wire a very complicated magnetic seolotropy. There is produced in 

 the wire a particular kind of helical magnetisation, with a particular twist and magnetic 

 seolotropy associated with it. Eep resenting this particular combination by the symbol 

 ( + C, 4- H), we may suppose one of two things done. We may superpose either the circular 

 magnetising force ( — 2C) or the longitudinal magnetising force ( — 2H), giving rise 

 respectively to the combination ( — C, +H) or ( + C, -H). The seolotropy being of a 

 helical character, the effect of applying the force — 2C will in general differ from the 

 effect of applying the force — 2H. The pitch, so to speak, of the magnetisation helix 

 due to the combination ( + C, + H) will depend on the relative values of C and H ; and 

 this helical seolotropy will in general have different geometrical relationships to the 

 circular or longitudinal magnetising forces that build up the resultant magnetising force. 

 Bearing in mind the great difference already referred to between the magnetic effects 

 of the very simple combinations + b — b and — b + b, we should expect as funda- 

 mental a difference to exist between the magnetic effects of the more complex combina- 

 tions ( — C, + H) and ( + C, — H). What these magnetic effects are we know only in a 

 rough way. In the present instance, we are studying them by their accompanying 

 strain effects which show forth after their own fashion the predominating influence of the 

 magnetic after-effect. 



10. Complete Cycles. — So far we have considered only the total twists produced by 

 reversal of either of the component forces. But clearly, if the phenomenon of the 

 magnetic after-effect is at all evident, we should expect to find it declaring itself in the 

 course of any one cyclic variation. With a given combination of magnetising forces, 

 there are two ways of going through a cycle according to the particular component we 

 choose as the variable. A few examples will suffice to indicate the law by which the 

 changes of twist lag behind the changes of current or field. 



In Table IV. four complete cycles are given, and the corresponding curves will be 

 found in Plate III. The first three sets are for cyclic variations of the field ; the fourth 

 is for cyclic variations of the current. In each case the second and third columns contain 

 the twists, which should be read down in the former and up in the latter, if it is desired 

 to go through the cycle in the order of experiment. By this arrangement the after- 

 effect is evident at a glance. 



