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CXIII. On Magnetic Hysteresis. 

 To the Editors of the Philosojjhical Magazine. 

 Gentlemen, — 

 HE interesting article in your September. issue in which 



Professor S. P. Thompson applies the Fourier analysis 

 to Hysteresis Corves in the magnetization of iron doses 

 with some observations regarding the nature and effects of 

 hysteresis to which I think exception must be taken. He 

 remarks (p. 436) : "Hysteresis is commonly regarded as an 

 irreversible process and as such involving a degradation of 

 energy into heat. But in view of the present analysis of the 

 hysteresis loop it is necessary to revise this opinion.'' 

 I cannot see why. Hysteresis in the magnetization of iron 

 means a lagging of the magnetism behind the magnetizing 

 force when the magnetizing force is made to suffer any 

 change. This lagging causes each stage of the process to be 

 irreversible and involves a deg/adation of energy into heat. 

 The area enclosed by the curve of magnetism and magne- 

 tizing force measures the energy so dissipated in a cyclic 

 process. These fundamental facts are in no way affected by 

 the analysis, and it is not apparent that any revision of ideas 

 is required. Neither can one accept the statement that 

 " the energy waste does not involve or produce any pheno- 

 menon of lag." It is, on the contrary, the lag of magnetism 

 behind magnetizing force that involves waste of energy. 



Professor Thompson seems to have in mind, when he 

 speaks of lag, the lag of current behind impressed electro- 

 motive force in a circuit such as that of a choking coil, and 

 he is at pains to point out that " hysteresis does not cause 

 any lag in the current." But to suggest that it might would 

 be to put the cart before the horse. It is the current that 

 represents the magnetizing force, and the lag which hysteresis 

 involves is the lag of something else (namely the magnetism) 

 behind the current, not any lag of the current behind some- 

 thing else. 



Yours faithfully, 



J. A. EwiNG. 

 Froghole, Edenbridee, Kent, 

 5 Nov., 1910. 



