Abstract: In combination: an air core coil; a passive detector connected to said coil for giving a first output in response to change in the ambient magnetic field due to movement of a magnetic body near the coil; an active detector connected to the same coil and operable to create a local magnetic field about the coil and to give a second output in response to changes in the local field due to the movement of the body; apparatus energizing the active detector in response to the output of the passive detector; and apparatus giving a further output in response to simultaneous outputs from both the detectors.