Theory of Induced Magnetism. 207 



mutual attraction by the shortest path." * But "Wiedemann 

 expressly postulates a frictioiial resistance to rotation, which 

 will prevent this arrangement from being more than approxi- 

 mately attained, and which may be more or less overcome by 

 vibration f. 



I lately commented on the fact that soft iron and other 

 magnetic metals (notably nickel under particular conditions 

 of strain J) show a remarkably close approach to instability at 

 certain stages in the magnetizing or demagnetizing process §. 

 When the magnetic force reaches a particular value, the rate 

 of change of magnetism with respect to change of force may 

 become enormous. Referring to this in a paper which has 

 just been published ||, Mr. A. E. Kennelly has reverted to the 

 idea of chains of magnetic molecules held together by the 

 inter-molecular magnetic forces, and contends that when such 

 a chain is ruptured by applying a sufficiently strong external 

 magnetic force it will fall to pieces throughout, and the mole- 

 cular magnets which compose the chain will take their align- 

 ment suddenly. He accordingly sketches what he calls a 

 " chain-theory " of magnetization and an adaptation of the 

 theory of Hughes, in which, however, he postulates an elastic 

 resistance to the rotation of the molecules in addition to the 

 constraint afforded by their mutual magnetic forces. Mr. 

 Kennelly's remarks are highly interesting and suggestive ; 

 but I do not think (for reasons which will appear immediately) 

 that the notion of closed magnetic chains can be maintained 

 as a general account of the molecular structure of unmagne- 

 tized iron. 



I have experimented on the subject by making a model 

 molecular structure consisting of a large number of short 

 steel bar-magnets, strongly magnetized, each pivoted like a 

 compass-needle upon a sharp vertical centre and balanced to 

 swing horizontally. We cannot readily imitate in a model 

 the two degrees of rotational freedom possessed by the axes of 

 actual molecular magnets, but a group of magnets swinging 

 in one plane gives a sufficiently good general idea of the nature 

 of the equilibrium w T hich is brought about by inter-molecular 

 forces, and the manner in which that equilibrium is disturbed 



* Hughes, he. cit. 



t Wiedemann, Phil. Mag. July 1886, p. 52 ; Elektricitat, vol. iii. 

 §§ 784-785. 



X See a paper by H. Nagaoka, Journal of the Science College of the 

 University of Tokio, vol. ii. (1888) p. 304. 



§ Journal of the Institution of Electrical Engineers, No. 84 (1890) 

 pp. 38-40. 



|| The Electrician, June 7 and 13, 1890, 



Q2 



