METAL DETECTOR FOR IDENTIFYING AND DISCRIMINATING BETWEEN OBJECTS OF DIFFERENT SIZE, SHAPE, ORIENTATION AND FERROUS CONTENT AND INCLUDING AN AUTO NULLING CIRCUIT

A metal detector comprising a sensor coil positioned relative to an excited field coil for minimum mutual inductive coupling therebetween to provide a null output from the sensor coil in the absence of objects to be detected. The coils are rigidly secured in a lightweight epoxy casting to preclude deformation of the coils and relative displacement therebetween under varying environmental conditions to preclude drift of the null output. Low frequency excitation on the order of a few hundred up to a few thousand hertz is applied to the field coil to obtain a high degree of insensitivity to moisture conditions existing in the vicinity of the detector for further maintaining constancy of the null output. An auto-nulling circuit for automatically compensating for any long term drift of the detector output is also optionally included.