Magnetic metal detector utilizing a magnetic bridge formed with permanent magnets and a hall effect sensor

Claim 1

APPARATUS FOR DETECTING THE PRESENCE OF MAGNETIC MEMBERS COMPRISING: A MAGNETIC BRIDGE CIRCUIT INCLUDING A FIRST MAGNETIC LOOP FORMED OF AT LEAST ONE PERMANENT MAGNET MEMBER AND A PLURALITY OF INTERCONNECTED FERROMAGNETIC MEMBERS ARRANGED IN A SUBSTANTIALLY CLOSED LOOP, SAID FERROMAGNETIC MEMBERS BEING FORMED OF A MATERIAL HAVING HIGH MAGNETIC PERMEABILITY BUT LOW RETENTIVITY; A SECOND MAGNETIC LOOP FORMED OF AT LEAST A PAIR OF PERMANENT MAGNETIC MEMBERS HAVING NORTH AND SOUTH POLES RESPECTIVELY DISPOSED IN ALIGNMENT WITH ONE FERROMAGNETIC MEMBER THAT IS COMMON TO BOTH SAID FIRST AND SECOND MAGNETIC LOOPS, SAID SECOND MAGNETIC LOOP HAVING A GAP THEREIN CREATING A GREATER MAGNETIC RELUCTANCE IN SAID SECOND MAGNETIC LOOP THAN THE MAGNETIC RELUCTANCE OF SAID FIRST MAGNETIC LOOP, SAID MAGNETIC MEMBERS OF SAID RESPECTIVE MAGNETIC LOOPS BEING SO CONSTRUCTED AND ARRANGED AS TO PRODUCE SUBSTANTIALLY EQUAL AND OPPOSITE MAGNETIC FLUX THROUGH SAID COMMON MEMBER THEREBY MAKING THE NORMAL EFFECTIVE FLUX FIELD IN SAID COMMON MEMBER SUBSTANTIALLY EQUAL TO ZERO; A HALL VOLTAGE GENERATOR POSITIONED IN SAID FLUX FIELD OF SAID COMMON FERROMAGNETIC MEMBER, SAID HALL GENERATOR INCLUDING MEANS FOR APPLYING A CONTROL CURRENT THERETO AND HALL VOLTAGE OUTPUT TERMINALS; AND MEANS CONNECTED TO SAID HALL VOLTAGE TERMINALS FOR INDICATING A VOLTAGE OUPUT FROM SAID HALL GENERATOR WHEN A MAGNETIC MEMBER IS DISPOSED IN THE VICINITY OF SAID GAP IN SAID SECOND MAGNETIC LOOP THEREBY REDUCING THE RELUCTANCE OF SAID SECOND MAGNETIC LOOP AND PRODUCING A CORRESPONDING INCREASE IN FLUX FIELD MAGNITUDE THROUGH SAID COMMON MEMBER AND A HALL VOLTAGE OUTPUT CORRESPONDING TO THE RELATIVE POSITION OF SAID MAGNETIC MEMBER WITH RESPECT TO SAID GAP IN SAID SECOND MAGNETIC LOOP.