On the Magnetic Torsion of Iron and Nickel Wires. 251 



Earth has become almost altogether solid, the moment ot 

 inertia of the shell with respect to its polar axis must be 

 always greater than the moment of inertia for its equa- 

 torial axis. 



The tendency of the fluid nucleus to increase in ellipticity 

 might produce a result worthy of examination by volcanolo- 

 gists, namely, a possible increase in the development of vol- 

 canic phenomena in equatorial as compared to polar regions 

 with the progressive solidification of the Earth up to a certain 

 point. Until the thickness of the shell has become very great, 

 recent periods should exhibit a greater development of vol- 

 canic energy towards the equator than towards the poles, as 

 compared to remote epochs. 



XXIX. On the Magnetic Torsion of Iron and Nickel Wires. 

 By Shelfobd Bidwell, M.A., F.R.S.* 



IN a paper published in the Phil. Mag. for July 1886, p. 50, 

 Prof. Gr. Wiedemann refers to his well-known experi- 

 mental researches into the relations between the torsion and 

 magnetization of iron wires. Let a longitudinally magne- 

 tized wire NS, be fixed at the south end S, the other end N 

 being free, and let a battery current be passed through it 

 from S to N, then the free end will be observed to twist in 

 the direction of the hands of a clock as seen from the fixed 

 end. 



Maxwell f explains the phenomenon thus. The wire being- 

 magnetized both circularly and longitudinally, the resultant 

 magnetization is in the direction of a right-handed screw round 

 the wire. Now Joule found that an iron bar when longi- 

 tudinally magnetized was increased in length, its transverse 

 dimensions being at the same time contracted. We should 

 expect therefore that a spirally magnetized wire would expand 

 in the direction of the magnetization and contract in directions 

 at right angles to the magnetization. And thus the twisting 

 would be produced. 



This explanation assumes that the torsional effect is simply 

 secondary to and dependent upon the phenomena observed 

 by Joule, and Prof. Wiedemann, for reasons given in his 

 paper, appears not to be satisfied with it. 



Some additional light may perhaps be thrown on the 

 subject by a few experiments which I made at the beginning 

 of the present year. They were intended to be merely of a 



* Communicated by the Author, 

 t ' Electricity/ ii. § 448. 

 S2 



