494 Sir J. A. Ewing on a Ntw 



being turned into alignment by a sufficiently strong external 

 field, and that the control of the Weber elements — which 

 resists their turning and gives rise to the phenomena of 

 hysteresis — is due entirely to magnetic forces. When they 

 are caused to turn there is first a small stable deflexion, then 

 a breaking away from the position of stability and a falling 

 over, through an unstable phase, into a new position of 

 stability. In this essential characteristic the new model 

 resembles the old one. Bat in the old model the only 

 magnetic forces that contributed to the control of any one 

 Weber element were the forces between it and other Weber 

 elements. In the new model a great part of the magnetic 

 control is due to forces between the Weber element and 

 other portions of the same atom. The Weber element — the 

 thing that turns — is probably only a small part of the atom*. 

 Before describing the new model it is necessary to point 

 out in what respect the old one fails. The old model 

 consisted entirely of pivoted magnets which represented the 

 Weber elements in the atoms of a crystal, say of iron. 

 Their mutual magnetic forces made them form rows. When 

 a weak external field was applied, the magnets in any row 

 that lay more or less transverse to it underwent a small 

 stable deflexion, corresponding to the initial stage of magnet- 

 ization which the late Lord Rayleigh showed to be quasi- 

 elastic. When the field was sufficiently increased the rows 

 broke up and other rows were formed in a more favourable 

 orientation. This corresponds to the "steep " stage in the 

 magnetizing curve. Now it is known that in ordinary iron 

 barely one per cent, of the whole magnetism of saturation is 

 acquired in the quasi-elastic stage before the effects of 

 hvsteresis set in. To conform to this condition, the magnets 

 of the model must have only a very narrow range of stable 

 deflexion, and consequently they have to be set ve*ry near 

 together, with the result that in the old model their mutual 

 control became excessive. A calculation of the force 

 required to break up rows of pivoted magnets, of atomic 

 dimensions, when set near enough together to satisfy the above 

 condition, showed it to be many thousands of times greater 

 than the force which is actually required, in iron, to reach 

 the steep part of the curve. The original mod^l, therefore 

 failed quantitatively, and a model had to besought for which 

 would exhibit similar qualitative features but would provide 

 a far weaker control, while still allowing no more than a 

 very narrow range of stable deflexion. This is arrived at as 

 follows. 



* Cf. A. H. Compton andO. Rognley, Phys. ttev. xvi. p. 464 (1920). 





