Electric current is used to create a longitudinal magnetic field in a piece of magnetic material.
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. This is the most desirable condition for a reliable inspection. Fatigue and stress corrosion cracks are examples of cracks usually open to the surface.