Figure 15-9.-Coil creates a longitudinal field to show crack in a part.

the part and induce a longitudinal magnetic field. Inspection of a cylindrical part with longitudinal magnetism is shown in figure 15-9. If there is a transverse discontinuity in the part, such as that in the illustration, small magnetic poles are formed on either side of the crack. These poles will attract magnetic particles, forming an indication of the discontinuity. Compare figure 15-9 with figure 15-5, and note that in both cases a magnetic field has been induced in the part that is at right angles to the defect. This is the most desirable condition for a reliable inspection.

ALTERNATING CURRENT. -The use of alter-nating current (ac) in magnetic particle inspection is recommended only for the detection of surface discontinuities, which comprise the majority of service-induced defects. Fatigue and stress corrosion cracks are examples of cracks usually open to the surface. Alternating current, which must be single phase when used directly for magnetizing purposes, is taken from commercial power lines or portable power sources, and is usually 50 or 60 hertz.

DIRECT CURRENT. -Direct current (dc) mag-netizes the entire cross section more or less uniformly in the case of longitudinal magnetization. Magnetic fields produced by direct current penetrate deeper into apart than fields produced by alternating current, which makes it possible to detect subsurface discontinuities. Generally, direct current is used with wet magnetic particle methods. In the presence of dc fields, dry powder particles behave as though they were immobile, tending to remain wherever they happen to land on the surface of a part. This is in contrast to what happens with dry powder particles in the presence of ac fields. In these fields, the particles have mobility on a surface due to the pulsating character of the fields. Particle mobility aids considerably the formation of particle accumulations (indications) at discontinuities.