Distribution and Intensity
the length of a component is several times larger than its diameter,
a longitudinal magnetic field can be established in the component.
The component is often placed longitudinally in the concentrated
magnetic field that fills the center of a coil or solenoid. This
magnetization technique is often referred to as a "coil shot."
magnetic field travels through the component from end to end with
some flux loss along its length as shown in the image to the right.
Keep in mind that the magnetic lines of flux occur in three dimensions
and are only shown in 2D in the image. The magnetic lines of flux
are much more dense inside the ferromagnetic material than in air
because ferromagnetic materials have much higher permeability
than does air. When the concentrated flux within the material
comes to the air at the end of the component, it must spread out
since the air can not support as many lines of flux per unit volume.
To keep from crossing as they spread out, some of the magnetic
lines of flux are forced out the side of the component.
a component is magnetized along its complete length, the flux
loss is small along its length. Therefore, when a component is
uniform in cross section and magnetic permeability, the flux density
will be relatively uniform throughout the component. Flaws
that run normal to the magnetic lines of flux will disturb the
flux lines and often cause a leakage field at the surface of the
When a component with considerable length is magnetized
solenoid, it is possible to magnetize only a portion
of the component. Only the material within the solenoid and about
the same width on each side of the solenoid will be strongly magnetized.
At some distance from the solenoid, the magnetic lines of force
will abandon their longitudinal direction, leave the part at a
pole on one side of the solenoid and return to the part at a opposite
pole on the other side of the solenoid. This occurs because the
magnetizing force diminishes with increasing distance from the
solenoid. As a result, the magnetizing force may only be strong
enough to align the magnetic domains within and very near the
solenoid. The unmagnetized portion of the component will not support
as much magnetic flux as the magnetized portion and some of the
flux will be forced out of the part as illustrated in the image
below. Therefore, a long component must be magnetized and inspected
at several locations along its length for complete inspection
- An electrically energized coil of insulated wire, which produces
a magnetic field within the coil.