Abstract: To detect a marker element, a transmitter generates an alternating magnetic field. A receiver arranged in this field generates an electrical signal having a waveform corresponding to the magnetic flux density of the alternating magnetic excitation field superimposed by peaks produced upon reversal of the magnetic state of the marker element. The position of these peaks, relative to the alternating magnetic excitation field, is determined, and if different from half the period duration of the alternating magnetic excitation field by less than a predetermined value, a first state of the marker element may be detected. Otherwise, it may be determined that, either, no marker element is present in the alternating magnetic excitation field, or that the marker element is in a second state different from the first state.