_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 of the bar must pass back
through the air to the other end of the bar, as indicated by dotted
lines and arrows. The path followed by the magnetic lines of force is
called the _magnetic circuit_, and, therefore, the magnetic circuit of
the magnet shown in Fig. 90 is composed partly of iron and partly of
air. From what has been said concerning the relative permeability of
air and of iron, it will be obvious that the presence of such a long
air path in the magnetic circuit will greatly reduce the number of
lines of force that a given magnetizing force can set up. The presence
of an air gap in a magnetic circuit has much the same effect on the