If a bar with a length much greater than its diameter is considered, then longitudinal
defects would be detected by current flow and transverse and circumferential defects
by the indirect method of an encircling coil or magnetic flux flow.
Subjects in which cracks radiating from a hole are suspected can be tested by means
of the threading bar technique, whereby a current carrying conductor is passed
through the hole and the field induced is cut by any defects. Detection of longitudinal
defects in hollow shafts is a typical application of the threader bar technique.