Static magnetic field metal detector

Claim 1

Apparatus responsive to passage of a metal object through one or the other of discrete contiguous portions of a region of space comprising: first and second elongate magnetic-field generation means for producing respective first and second unidirectional magnetic fields in said discrete contiguous portions of said region of space, said first unidirectional magnetic field being opposite in sense to said second unidirectional magnetic field, said first elongate magnetic field-generation means comprising first elongate permanent magnet means having first and second elongate substantially parallel-disposed magnetic poles at one face thereof, said second elongate magnetic-field generation means comprising second elongate permanent magnet means having third and fourth elongate substantially parallel-disposed magnetic poles at one face thereof, said first, second, third and forth magnetic poles being characterized by production of said first unidirectional magnetic field of one sense adjacent said one face of said first elongate permanent magnet means and said second unidirectional magnetic field of opposite sense adjacent said one face of said second elongate permanent magnet means, said unidirectional magnetic fields being substantially equal in magnitude, multiple conductor magnetic field pick-up coil means disposed substantially equally in stationary relation in said first and second unidirectional magnetic fields for providing a substantial detection signal in the presence of said object passing adjacent said first or adjacent said second elongate magnetic field-generation means while affording substantial cancellation of spurious magnetic field responses, and utilization means coupled to said multiple conductor magnetic field pick-up coil means thereby responsive to the passage of said metal object through one or the other of said discrete contiguous portions of said region of space.