Magnetic flux lines must run in a direction such that they are interrupted at a defect, causing a flux leakage. So, in order to detect defects, the flux lines should ideally be at 90° to their principal direction. In order to locate all defects within a specimen, we need to generate magnetic fields in two directions at 90° to one another. These are normally:
In Fig.1 the magnetic lines of force are longitudinal in a bar and thus the bar has magnetic poles. Transverse flaws will easily show, but longitudinal defects such as seams, which are very straight, will not. However, it is accepted that flaws up to 45° to the flux lines will also be shown. In fact, longitudinal flaws having a transverse component, such as jagged cracks, will almost certainly show.
The longitudinal magnetising field in the bar is now replaced by a longitudinal current, which creates a magnetic field at 90° to itself. In fact, the current has produced a circular non-polar field around the bar. Under normal circumstances the circular field is not detected, due to it having no external poles, but a longitudinal surface flaw at 90° creates a flux leakage, creating miniature poles and is thus detectable with magnetic particles. Figure 2 shows the effect of flaw orientation in a circularly magnetised bar.