a method and apparatus for use with a measuring and / or control device having at least one sensor based on eddy current induction and movable over a test piece and supplied with at least two carrier frequency signals . a determination is made on one of the carrier frequency signals with respect to those signal changes obtained at the transistion from measuring against the test piece to measuring against essentially empty space . when such signal changes occur an edge signal is generated which indicates that the measuring and / or control device has passed over an edge of the test piece .