ver, 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
_magnetization curves_, it is easy to determine other data from which
so-called permeability curves may be plotted. In permeability curves
the total magnetization of the given pieces of iron are plotted as
abscissas, while the corresponding permeabilities are plotted as
ordinates.

[Illustration: Fig. 89. Magnetization Curve]

Direction of Lines of Force. The lines of force set up within the
core of a helix always have a certain direction. This direction always
depends upon the direction of the flow of current around the core. An
easy way to remember the direction is to consider the helix as grasped
in the right hand with the fingers partially encircling it and the
thumb pointing along its axis. Then, if the current through the
convolutions of the helix be in the direction in which the fingers of
the hand are pointed around the helix, the magnetic lines of force
will proceed through the core of the helix along the direction in
which the thumb is pointed.

In the case of a simple bar electromagnet, such as is shown in Fig.
90, the lines of force emerging from one end