121 
1921 - 22 .] On Models of Ferromagnetic Induction. 
space-lattice, the neighbours will be affected in such a way that the balance 
of forces between the fixed magnets in them will be upset. Each of the 
neighbours will find itself unsymmetrically surrounded ; the symmetry of 
magnetic forces within it will be disturbed, and the control of the Weber 
element will consequently be increased. This action will no doubt extend, 
in diminishing degree, through several layers of atoms near each foreign 
particle, and it will be irregularly distributed throughout the piece as a 
whole. Compared with pure metal, the piece will therefore lose some of 
its susceptibility, and also become less magnetically homogeneous, with the 
result that the curve of magnetisation will show more rounded outlines, as 
well as a higher coercive force. This agrees with what is observed. 
These remarks apply in general terms, whether the foreign atoms are 
present in mixture, or in solid solution, or in combination with those of the 
ferromagnetic metal. In a chemical compound, say of iron, the distortion 
of the iron atom that comes of its being unsymmetrically surrounded may 
cause its magnetic properties to differ widely from those of an atom of 
uncombined iron, and may be associated with the magnetic aeolotropy 
which is a well-known feature of the crystals of certain iron compounds. 
With the model shown in fig. 15 (PI. II) we can imitate conditions of extreme 
seolotropy such, for instance, as Weiss found in crystals of pyrrhotite.* 
Suppose that in the model one pair of opposite fixed magnets are advanced 
until they touch one pair of opposite poles in the turning octet which 
represents the Weber element. One axis of*the octet (namely, a trigonal 
axis) thereby becomes fixed, and it can turn only by rotation about that 
axis. It turns through 120° in passing from one position of stability to 
another. This state of things is illustrated in the photograph (fig. 18, PI. II), 
where the central pivot is removed and the octet is supported by contact of 
poles in the axis AB, about which it can turn, two of the ball ends being 
slightly cupped to keep it from falling out of place. The behaviour of the 
model now resembles that of pyrrhotite, which takes up magnetic induction 
readily in one plane but not in a direction perpendicular to that plane. 
The other fixed magnets may at the same time be adjusted to exhibit 
differences of susceptibility along different axes in the plane of magnetisa- 
tion. Such differences were in fact observed in pyrrhotite by Weiss. His 
experiments showed that when a pyrrhotite crystal was rotated about its 
non-magnetic axis in a constant field, acting in the plane of magnetisation, 
there were more or less abrupt magnetic changes at intervals of 60°. 
Inspection of the model suggests that these may be consequences of the 
turning of the Weber element about one of its trigonal axes. 'J'he project- 
* Jour, de Phys., iv, p. 469, 1905. 
