Semi finished products, such as wires, bars and tubes are tested for local crack-like and void-like flaws using circumferential through-type coils. A crack disturbs the propagation of the eddy currents generated in the test material. This produces the related flaw signal in the receiver coil.

The differential through-type coil detects void flaws and transverse flaws with high sensitivity and detects longitudinal flaws according to their depth gradient. The absolute through-type coil indicates serious longitudinal flaws, e.g. unwelded tubes, over their entire length.