Cover the frequency range of a few Hz up to approx. 1 kHz, usually with absolute coils. These are used for the determination of integral, technological material characteristics such as joints, hardness, penetration hardness and layer thicknesses.
Using magneto-inductive methods, the core and surface can be selectively examined by calibration of the frequency. Depending on the test application, monofrequency or multifrequency (serial or simultaneous) are stimulated.