Electricity and Magnetism.
Electric current can be used to create or induce magnetic fields in parts made of ferromagnetic materials. Magnetic
lines of force are always aligned at right angles (90°) to the direction of electric current flow. It is possible to control
the direction of the magnetic field by controlling the direction of the magnetizing current. This makes it possible to
induce magnetic lines of force so that they intercept defects at right angles.
Magnetic attraction can be explained using the concept of flux lines or lines of force. Each flux line forms a closed
continuous loop, which is never broken. For a circularly magnetized object, the flux lines are wholly contained in the
object (ideal case). No external magnetic poles are present and therefore there is no attraction for other ferromagnetic
objects. For a longitudinally magnetized object, the flux lines leave and enter at magnetic poles. The flux lines always
leave a ferromagnetic material at right angles to the surface. They always seek the path of least resistance, i.e.
maximum permeability and minimum distance. When a piece of soft iron is placed in a magnetic field it will develop
magnetic poles. These poles will be attracted to the poles of the magnetic object that created the initial field. As it
approaches closer to the source original field, more flux lines will flow through the piece of iron, thus creating stronger
magnetic poles and further increasing the attraction. This concentrates the lines of flux into the easily traversed (high
permeability) iron path rather than the alternative low permeability air paths. This is magnetic attraction and is the
reason magnetic particles concentrate at leakage fields. The leakage field is established across an air gap of relativity
low permeability at the discontinuity. Since they offer a higher permeability path for the flux lines, the magnetic
particles are drawn to the discontinuity and bridge the air gap to the extent possible.
Effects Of Flux Direction.
The magnetic field must be in a favorable direction, with respect to a discontinuity, to produce an indication. When the
flux lines are parallel to a linear discontinuity, the indications formed will be weak. The best results are obtained when
the flux lines are perpendicular (at right angles) to the discontinuity. Note: When an electrical magnetizing current is
used, the best indications are produced when the path of the magnetizing current is parallel to the discontinuity.
A circular magnetic field always surrounds a current carrying conductor, such as a wire or a bar (see Figure 3-8). The
direction of the magnetic lines of force (magnetic field) is always at right angles to the 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 right 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. This is called the right hand
Figure 3-8. Magnetic Field Surrounding an Electrical Conductor.
Since metals are conductors of electricity, an electric current passing through a metallic part creates a magnetic field as
shown in Figure 3-9. The magnetic lines of force are at right angles to the direction of the current. 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.