Metal detector including a metal screening for producing a secondary magnetic field to reduce the metal free zone

A metal detector comprising a coiled conductor defining a channel extending therethrough, the coiled conductor being operative to be connected to an a.c. power supply so as to provide an a.c. generated primary electromagnetic field, the detector further comprising means for generating a secondary magnetic field for nullifying the primary field beyond a distance from the coiled conductor, the means for generating the secondary field being stationary, and at least a part of the means for generating the secondary field being located within the primary field and being located adjacent the coiled conductor, characterized in that the coiled conductor is located within a metal enclosure and at least a part of the means for generating the secondary magnetic field extends between the coiled conductor and the metal enclosure, the metal enclosure having apertures in opposed walls thereof, the apertures being aligned with the coil channel such that a material to be subjected to metal detection can be passed therethrough, the position of the means for generating the secondary magnetic field being such that it does not interfere with the passage of said material along the path of travel through the enclosure apertures and the coil channel.