Patent ID: 7113092

Claim:
An apparatus for detecting a ferromagnetic object comprising a passive primary sensor comprising first and second magnetic sensors, the primary sensor adapted to measure an ambient magnetic field within a localized volume of space defined by a zone of sensitivity of the first and second magnetic sensors and to produce a corresponding measurement signal, a secondary, non-magnetic, sensor adapted to detect the movement of objects in the vicinity of the primary sensor, a signal processor arranged in communication with the primary and secondary sensors, and a warning device operable by an output from the signal processor, the warning device adapted to provide within the vicinity of the primary sensor at least one of an audible warning and visible warning, wherein the signal processor is configured to identify temporal variations in the measurement signal due to the movement of a ferromagnetic object within the an ambient magnetic field and to correlate the identified temporal variations in the measurement signal with movement of objects detected by the secondary, non-magnetic sensor, and to provide an output indicative of the presence of a ferromagnetic object in the vicinity of the primary sensor only in the presence of a correlation there-between.