Extended range mutual inductance distance gauge

Claim 1

MEANS FOR GAUGING DISTANCE D FROM A MEASUREMENT LOCATION TO A CONDUCTIVE SURFACE, COMPRISING A SENSING TRANSFORMER HAVING PRIMARY AND SECONDARY SENSING COILS WHICH IN USE ARE POSITIONED AT THE MEASUREMENT LOCATION, WHEREBY THEIR MUTUAL INDUCTANCE IS RELATED TO DISTANCE D, A REFERENCE TRANSFORMER COMPRISING PRIMARY AND SECONDARY COILS HAVING A CONSTANT MUTUAL INDUCTANCE, DRIVE MEANS FOR APPLYING ALTERNATING DRIVER CURRENT TO SAID PRIMARY COILS IN SERIES, SAID SECONDARY COILS BEING SERIALLY CONNECTED WHEREBY TO PRODUCE A RESULTANT OUTPUT VOLTAGE PROPORTIONAL TO THE DIFFERENCE BETWEEN SAID MUTUAL INDUCTANCES MULTIPLIED BY THE PRODUCT OF DRIVER CURRENT AND DRIVER CURRENT FREQUENCY, FEEDBACK LOOP CIRCUIT MEANS INCLUDING SAID DRIVE MEANS AND OPERABLE IN RESPONSE TO DEVIATIONS OF SAID RESULTANT OUTPUT VOLTAGE FROM A PREDETERMINED CONSTANT VALUE WHEREBY TO REGULATE SAID RESULTANT OUTPUT VOLTAGE AT A SUBSTANTIALLY CONSTANT VALUE BY CONTROL OF SAID DRIVE MEANS, WHEREBY THE PRODUCT OF DRIVER CURRENT AND DRIVER CURRENT FREQUENCY IS MADE PROPORTIONAL TO THE RECIPROCAL OF THE DIFFERENCE BETWEEN SAID MUTUAL INDUCTANCES, AND INDICATOR MEANS FOR MEASURING THE OUTPUT OF SAID DRIVE MEANS.