unit cross-section of air. Evidently for air B = H, hence µ becomes
unity.

The permeability of air is always a constant. This means that whether
the magnetic density of the lines of force through the air be great or
small the number of lines will always be proportional to the
magnetizing force. Unfortunately for easy calculations in
electromagnetic work, however, this is not true of the permeability of
iron. For small magnetic densities the permeability is very great, but
for large densities, that is, under conditions where the number of
lines of force existing in the iron is great, the permeability becomes
smaller, and an increase in the magnetizing force does not produce a
corresponding increase in the total flux through the iron.

Magnetization Curves. This quality of iron is best shown by the curves
of Fig. 89, which illustrate the degree of magnetization set up in
various kinds of iron by different magnetizing forces. In these curves
the ordinates represent the total magnetization =B=, while the abscissas
represent the magnetizing force =H=. It is seen from an inspection of
these curves that as the magnetizing force =H= increases, the intensity
of flux also increases, but at a gradually lessening rate, indicating a
reduction in permeability at the higher densities. These curves are also
instructive as showing the great differences that exist between the
permeability of the different kinds of iron; and also as showing how,
when the magnetizing force becomes very great, the iron approaches what
is called _saturation_, that is, a point at which the further increase
in magnetizing force will result in no further magnetization of the
core.

From the data of the curves of Fig. 89, which are commonly called