124 Proceedings of the Poyal Society of Edinburgh. [Sess. 
structure : the cube-centred space-lattice persists at temperatures above 
as well as below the magnetic critical point.* The model will serve to 
illustrate how a complete loss of magnetic quality may occur without 
apparent change in the structure of the space-lattice or even in that of the 
atom itself. 
31. In conclusion, it may be useful to attempt a summary of chief 
points, distinguishing those that are well established from those that are 
conjectural. 
Magnetic induction in ferromagnetic solids is characterised not only by 
its relatively great amount but by the phenomena of saturation and 
hysteresis. It cannot be doubted that the process occurs through the 
turning of “Weber elements” which possess magnetic moment. The thing 
that turns is not the molecule nor the atom, but something within the atom. 
In a pure magnetic metal — say iron in the normal state — each atom has 
in it a part possessing magnetic moment, f which turns in response to an 
impressed magnetic field. Its turning is opposed by controlling forces 
which give it stability in a number of different angular positions. When 
it is displaced by an impressed field from any one of these positions, its 
deflection is at first reversible ; but if a narrow range of stable deflection 
be exceeded, the turning element becomes unstable : it breaks away and 
falls over, with dissipation of energ}", into another position of stable 
equilibrium. 
Thus when a field is first applied to a previously unmagnetised piece, 
the initial deflections of the Weber elements are reversible : hence within 
a very narrow range the magnetisation is quasi-elastic, and there will be 
no residual magnetism if the field be withdrawn. Similarly, at any stage 
in a magnetising process a change from increasing to decreasing field, or 
vice versa, is marked by a short reversible stage in the turning of the 
Weber elements. All these points are well established. 
In my theory of 1890 the stable and unstable stages were explicitly 
recognised, and it was pointed out that hysteresis arose from the falling 
over of the Weber element from one position of stability to another. 
Under conditions which prevent that falling over there is no hysteresis. 
* Log. cit., p. 315 et seq. “The so-called )8 iron has the same lattice as the a iron. . . . 
No difference has been found in the structure of iron below and above A2. At A3, how- 
ever, the atoms of iron are completely rearranged, and the iron passes from one crystal 
class into another.” The magnetic change occurs at the lower arrest point A2. 
t From known data as to the space-lattice of the iron crystal and the saturation limit 
of magnetisation in iron, it is easy to show that the moment of the Weber element in an 
atom of iron is very nearly 2 x 10~^® c.g.s. units. (See Proc. Roy. Soc., February 1922, 
vol. 100, A, p. 453.) 
