PHYSICS 191 



same volume of the Proceedings, Ewing points out one rather 

 unsatisfactory feature of his former well-known model, which 

 is made up of little magnets pivoted on uniformly spaced fixed 

 centres and all free to turn, but individually controlled by their 

 mutual magnetic forces. It concerns the stability of the rows 

 of magnets in the model when an external magnetic field is 

 applied. Actually when a slowly increasing magnetising field 

 acts on a piece of soft iron, the magnetisation it acquires during 

 the quasi-elastic stage which precedes the appearance of 

 hysteresis is a very small part, often less than i per cent., of 

 the intensity acquired at saturation. Hence the range of stable 

 deflection which the " Weber element " in an atom of iron can 

 undergo before instability occurs is very narrow, less than a 

 degree probably on each side of its initial direction. In con- 

 sequence, we must postulate very close juxtaposition of the 

 pivoted magnets in the model with but little clearance between 

 the poles. Nevertheless, employing both theoretical calcula- 

 tion and experimental verification, Ewing shows that such 

 a model fails to represent quantitatively the stages of magnetisa- 

 tion ; for it would necessitate an external field sufficiently 

 strong to rupture the arrangement of the atomic magnets, 

 which would be at least a million times as great as the field 

 which actually suffices to carry soft iron beyond the quasi- 

 elastic stage into the state when hysteresis begins to manifest 

 itself. Impressed by these considerations Ewing proposes a 

 new model which while retaining the idea of magnetic control, 

 escapes the excessive stabiHty of the old model and satisfies 

 the essential condition that the Weber element in each atom 

 may turn only through a very small angle before becoming 

 unstable. The idea is that the electrons in each atorn comprise 

 two groups, which by reason of rotatory motion in Hmited 

 orbits or of ring structure, constitute two magnetic systems ; 

 an inner one possessing magnetic moment and capable of turn- 

 ing in an applied field, and an outer which is to be regarded as 

 held approximately fixed relative to neighbouring atoms when 

 the atom is part of a solid body, and therefore takes its proper 

 place in the space-lattice of a crystal. The outer group behave 

 as a set of fixed directing magnets and so the model would be 

 one in which each model atom contained a Weber element 

 (the inner part) capable of turning, controlled, however, not by 

 neighbouring atoms, but by the remaining parts (outer) of its 

 own atom. To illustrate this idea Ewing has constructed 

 models with eight fixed magnets pointing along the diagonals 

 of a cube, their centres being at the corners of the cube, and an 

 inner part (representing the Weber element) which could be 

 either a simple pivoted magnet or an octet of poles set at the 

 corners of a cube (also pivoted at its centre) or a quartet of 



