Permeability of Iron and Steel. 85 
Let 0, or the rotation, represent the magnetic induction. 
It is clear that, however great C may be, it can never tend 
to increase @ beyond 180°, since this is the position in 
which the coil tends to place the needle independently of the 
torsion: this represents saturation. Further, C is proportional 
to the magnetic potential or magnetizing force, so that G iS 
proportional to w and Quen 6; so that p is represented 
approximately by a quantity proportional to the sine of an 
angle proportional to the induction. 
If the fundamental ideas of this illustration could be in 
any way applied to the behaviour of systems of molecules, it 
would be easy, by small modifications of the principal sup- 
positions, to take count of the smaller terms; but it seems 
quite impossible to carry out this application; and the illus- 
tration is only useful as showing the sort of circumstances 
which would have toe be imagined in order to obtain a 
formula of this sort from the hypothesis of molecular mag- 
nets. 
I will proceed to sketch another theory which appears to 
me more reasonable, and leads to forms of function by which 
we can represent the experiments very well. 
Suppose that every molecule of iron has one axis, and only 
one axis, through which magnetism can be transmitted. 
The molecules in an inert mass of iron are supposed to have 
their axes of transmission distributed in all directions. There 
will then be a very small proportion of molecules whose axes 
are so situated as to form continuous lines of transmissibility 
for magnetic lines of force proceeding in a given direction. 
Thus, for small magnetizing forces there will be a small trans- 
missibility or permeability. 
In the case in which molecules are so situated that the 
extremities of their axes do not join, the magnetic lines of 
force have to pass between the molecules as they would through 
ordinary space. We suppose that the transmissibility through 
the axes of the molecules is very great compared with that in 
ordinary space or in the interstices of the molecules. 
As the magnetic induction increases, couples are established 
by the action of the lines of force, which tend to move the 
axes of the molecules towards the direction of the lines of 
force. 
Consider a single molecule. Let the plane of the paper 
cut it in the plane containing its magnetic transmission axis 
and the axis of magnetization. ‘Then the lines of force will 
traverse the molecule somewhat in the way shown in the 
