117 
1921-22.] On Models of Ferromagnetic Induction. 
distribution in the pair of fixed magnets towards which it happens at 
any instant to point, in such a manner that in each of the four positions 
there is a margin of stability. It is possible that there may be an action 
in the atom corresponding in effect to this feature in the model, the 
result of which is to make the mutual forces between the attracting 
poles always a little stronger than the mutual forces between the 
repelling poles. 
24. In a ferromagnetic metal the space-grouping of the parts that 
are equivalent to fixed magnets in the shell of an atom is presumably 
subject to the symmetry that characterises crystals of the metal. Ferro- 
magnetic metals are cubic, and according to the X-ray analysis of A. W. 
Hull the space-lattice of iron is the centred-cube, in which each atom has 
eight nearest neighbours, set at the corners of a cube of which it is the 
centre.* It is natural to imagine a corresponding structure for the atom 
itself, in respect of the grouping of the fixed magnets. This consideration 
leads to a model (fig. 14, PI. II) with eight fixed magnets pointing along 
the diagonals of a cube, namely, the four fixed magnets of fig. 13 and 
other four in a plane at right angles to the first. In the model shown in 
fig. 1 4 they are held by a skeleton cubical structure of brass at the centre 
of which the pivot of the Weber element may be placed. By loosening a 
small set-screw each of the fixed magnets can slide in its support, in the 
direction of its length, so that its distance from the centre may be 
adjusted. The Weber element and its pivot are omitted in this photo- 
graph. If the Weber element is a simple bar magnet, as in fig. 13, it 
will turn by steps of 70° 32' in passing from one position of stability to 
another, and in the absence of an impressed field there are eight possible 
positions of stability. 
25. But the Weber element may itself consist of more than a simple 
magnet with two poles. It may, for example, be made up of an octet of poles 
set at the eight corners of a cube, constituting a magnetic system which 
turns as a whole and possesses magnetic moment, four poles on one face 
of the cube being of one name, and the opposite four of the other name. 
In the model atom which is shown in hg. 15 (PI. II) this structure is realised ; 
the Weber element is there formed by screwing ball-ended steel wires into 
the eight corners of a small cubic steel boss, and magnetising the whole 
to give opposite polarities on opposite faces. The least angle through 
which it turns in passing from one position of stability to another is 90°. 
When there is no impressed field there are twelve possible positions. In 
* Phys. Rev.^ ix, p. 84, 1917 ; x, p. 661. This is confirmed by A.. Westgren, Jour. 
Iron and Steel Inst., ciii, p. 303, 1921. 
