THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



MARCH 1887. 



XXV. Notes on Electricity and Magnetism. — III. On the 

 Behaviour of Iron and Steel under the Operation of Feeble 

 Magnetic Forces. By Lord Rayleigh, Sec. R.S.* 



[Plate II.] 



THE question whether or not iron responds proportionally 

 to feeble magnetic forces is of interest not only from 

 a theoretical point of view, but from its bearing upon the 

 actual working of telephonic instruments. Considerable dif- 

 ference of opinion has been expressed concerning it, several of 

 the best authorities inclining to the view that a finite force is 

 required to start the magnetization. Prof. Ewing remarksf : — 

 "As regards the hysteresis which occurs when the magnetism 

 of soft iron is changed, my experiments confirm the idea 

 already suggested by other observers, that when the molecular 

 magnets of Weber are rotated they suffer, not first an elastic 

 and then a partially non-elastic deflection as Maxwell has 

 assumed, but a kind of frictional retardation (resembling the 

 friction of solids), which must be overcome by the magneti- 

 zing force before deflection begins at all." In a subsequent 

 passage J Prof. Ewing treats the question as still open, re- 

 marking that though his curves suggest that the initial value 

 of k (the susceptibility) may be finite, they afford no positive 

 proof that it is not initially zero, or even negative. 



My attention was first called to the matter about a year 

 and a half ago in connection with the operation of iron cores 



* Communicated bv the Author, 

 t Phil. Trans. 1886, p. 526, § 5. 

 X L. c § 61. 

 Phil Mag. S. 5. Vol. 23. No. 142. March 1887. R 



