Figure 3-9. Magnetic Field in Part used as a Conductor.
Circular Magnetization with Inspection Equipment.
One method of creating or inducing a circular field in a part with stationary MPT equipment is to clamp the part
between two contact plates and pass current through the part as indicated in Figure 3-10. A circular magnetic field is
created within the part. If a longitudinally aligned crack or discontinuity exists within the part, a leakage field will be
established at the site of each crack or discontinuity. The leakage field will attract magnetic particles to form an
indication of the discontinuity.
Figure 3-10. Creating a Circular Magnetic Field in a Part
For hollow or tube-like parts, it is important to inspect both the inside and outside surfaces. When such parts are
circularly magnetized by passing the magnetizing current through the part, the magnetic field on the inside surface is
negligible. To understand this, consider that a circular field exists externally to the net current flow only, not internally
as well. To produce a magnetic field on both surfaces of the part, a separate conductor, such as a copper rod, is
positioned inside the hollow part. Since a circular magnetic field surrounds such conductors when an electric current is
passed through them, it is possible to induce a satisfactory magnetic field that is strongest on the hollow parts inside
surface. This situation is illustrated in Figure 3-11 and Figure 3-12.
Figure 3-11. Using a Central Conductor to Circularly Magnetize a Cylinder.