MAGNETISM AND TWIST IN IRON AND NICKEL. 509 



was found to exist also in the case of iron. By that time, however, the various effects 

 of twist on longitudinally magnetised iron and nickel wires had been elaborately 

 investigated by Mr Nagaoka, assistant in the Physical Laboratory, whose papers * 

 form a very valuable addition to our knowledge of the relations of magnetism and strain 

 in the magnetic metals. I shall have frequent occasion to refer to some of Mr Nagaoka's 

 results. 



Those earlier investigations suggested several distinct lines of research, some of which 

 I have myself worked out more or less fully during the spring of 1889. These have all 

 to do with wires circularly magnetised by currents passing along them. 



16. This phenomenon, namely, the production of longitudinal magnetic polarity in a 

 circularly magnetised wire by twisting of the same, is in a certain sense reciprocal to the 

 phenomenon discussed in the earlier paragraphs of the present paper. But consideration 

 will show that the reciprocity is more apparent than real. For, as will be seen immedi- 

 ately, the mechanical strains involved in the one case are huge compared to those involved 

 in the other. In the experiments now to be described, the wire is twisted to and fro 

 through angles far beyond the so-called limits of torsional elasticity. The elements of the 

 wire are sheared to an extent that must be physical torture as compared with the slight 

 molecular strains involved in the phenomenon of magnetic twisting. There is, however, 

 as emphasised by Wiedemann, a similarity in the two classes of phenomena, inasmuch 

 as the effects are opposite in iron and nickel. In iron, a current in the direction of 

 magnetisation twists the wire so that any straight line in it parallel to the axis becomes 

 a right-handed helix ; in nickel the twist is the other way. Then, again, when an iron 

 wire conveying a current is twisted so that a straight line in it parallel to the axis 

 becomes a right-handed helix, the longitudinal magnetic intensity produced is co- 

 directional with the current ; in nickel, on the other hand, it is anti-directional. For 

 the same truth, stated in a somewhat different way, see Wiedemann's paper already 

 referred to (Wied. Ann., vol. xxvii. p. 383). 



My object in the experiments now to be discussed was to study the manner in 

 which the general phenomenon varied in detail under different combinations of twists 

 and currents. Four nickel wires of different diameter, and two iron wires of the same 

 diameter, were experimented with. Each wire, when in use, was stretched horizontally 

 at right angles to the magnetic meridian ; and at a suitable distance from the western end 

 of the wire a small mirror magnetometer of the usual construction was adjusted in line 

 with the wire. The eastern end was attached to the twisting apparatus. This requires 

 no particular description more than to say that the twisting was effected by a gearing of 

 toothed wheels, so that it could be applied very steadily and gradually. The method of 

 experimenting consisted in studying in succession the effect of a series of cyclic twistings 

 of different range, each cyclic twisting being taken in combination with a succession of 

 currents of different values. After the wire had been subjected a sufficient number of 

 times to the cyclic twisting of the required range, a cycle was gone through more slowly, 



* Journal of Coll. of Sci., Imp. Univ., Japan, vols. ii. and iii. 

 VOL. XXXVI. PART II. (NO. 18). 4 G 



