Figure 15-3.-Magnetic field surrounding an electrical conductor.
Figure 15-4.—Magnetic field in part used as a conductor.
Figure 15-5.—Creating a circular magnetic field in a part.
direction of the magnetizing current. An easy way to
remember the direction of magnetic lines of force
around a conductor is to imagine that you are grasping
the conductor with your hand so that the extended thumb
points parallel to the electric current flow. The fingers
then point in the direction of the magnetic lines of force.
Conversely, if the fingers point in the direction of current
flow, the extended thumb points in the direction of the
magnetic lines of force.
Since a magnetic part is in effect a large conductor,
electric current passing through this part creates a
magnetic field in the same manner as with a small
conductor (fig. 15-4). The magnetic lines of force are at
right angles to the direction of the current as before. This
type of magnetization is called “circular magnetization”
because the lines of force, which represent the direction
of the magnetic field, are circular within the part.
To create or induce a circular field in a part with
stationary magnetic particle inspection equipment, the
part is clamped between the contact plates and current
is passed through the part, as indicated in figure 15-5.
This sets up a circular magnetic field in the part that
creates poles on either side of any crack or discontinuity
that runs parallel to the length of the part. The poles will
attract magnetic particles, forming an indication of the
discontinuity.
Figure 15-6.—Using a central conductor to circularly magnetize a
cylinder.
Figure l5-7.—Using a central conductor to circularly magnetize
ringlike parts.
Figure 15-8.—Magnetic field in a part placed in a coil.
On parts that are hollow or tubelike, the inside
surfaces are as important to inspect as the outside. When
such parts are circularly magnetized by passing the
magnetizing current through the part, the magnetic field
on the inside surface is negligible. Since there is a
magnetic field surrounding the conductor of an electric
current, it is possible to induce a satisfactory magnetic
field by placing the part on a copper bar or other
conductor. This situation is illustrated in figures 15-6
and 15-7. Passing current through the bar induces a
magnetic field on both the inside and outside surfaces.
LONGITUDINAL MAGNETIZATION.—Lon-
gitudinal magnetization is used for the detection of
circumferential discontinuities, which lie in a direction
transverse to or at approximately right angles to a parts
axis. Electric current is used to create a longitudinal
magnetic field in a piece of magnetic material. When a
part of magnetic material is placed inside a coil, as
shown in figure 15-8, the magnetic lines of force created
by the magnetizing current concentrate themselves in
15-7
