T.O. 33B-1-13-7Figure 3-9. Magnetic Field in Part used as a Conductor.3.1.6.6.3 CircularMagnetizationwithInspectionEquipment.One method of creating or inducing a circular field in a part with stationary MPT equipment is to clamp the partbetween two contact plates and pass current through the part as indicated in Figure 3-10. A circular magnetic field iscreated within the part. If a longitudinally aligned crack or discontinuity exists within the part, a leakage field will beestablished at the site of each crack or discontinuity. The leakage field will attract magnetic particles to form anindication of the discontinuity.Figure 3-10. Creating a Circular Magnetic Field in a Part3.1.6.6.3.1For hollow or tube-like parts, it is important to inspect both the inside and outside surfaces. When such parts arecircularly magnetized by passing the magnetizing current through the part, the magnetic field on the inside surface isnegligible. To understand this, consider that a circular field exists externally to the net current flow only, not internallyas well. To produce a magnetic field on both surfaces of the part, a separate conductor, such as a copper rod, ispositioned inside the hollow part. Since a circular magnetic field surrounds such conductors when an electric current ispassed through them, it is possible to induce a satisfactory magnetic field that is strongest on the hollow part’s insidesurface. This situation is illustrated in Figure 3-11 and Figure 3-12.Figure 3-11. Using a Central Conductor to Circularly Magnetize a Cylinder.
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