Instrument for measuring the thickness of a non-conducting film on a base metal with compensation for base resistivity and ambient temperature

Claim 1

AN INSTRUMENT FOR MEASURING THE THICKNESS OF A SUBSTANTIALLY NON-ELECTRICALLY CONDUCTING FILM ON A BASE METAL COMPRISING AN OSCILLATORY CIRCUIT INCLUDING AN INDUCTOR MOVABLE INTO PROXIMITY TO SAID FILM AND OPERABLE TO CHANGE THE FREQUENCY AND AMPLITUDE OF OSCILLATION OF SAID CIRCUIT IN THE SENSE OPPOSITE TO CHANGES IN THE RESISTIVITY OF THE BASE METAL AND TO CHANGE THE AMPLITUDE OF OSCILLATION OF SAID CIRCUIT IN THE SAME SENSE AS CHANGES IN THE SPACING OF SAID INDUCTOR FROM SAID BASE METAL WHILST CHANGING THE FREQUENCY OF OSCILLATION IN THE OPPOSITE SENSE TO SUCH CHANGES IN SPACING, A FEED-BACK LOOP ACROSS SAID OSCILLATORY CIRCUIT, SAID LOOP INCLUDING A PARALLEL INDUCTIVE-CAPACITIVE CIRCUIT IN SERIES WITH A CATHODE FOLLOWER TUBE, SAID PARALLEL INDUCTIVE-CAPACITIVE CIRCUIT BEING TUNED TO A FREQUENCY LOWER THAN THE FREQUENCY OF OSCILLATION OF SAID OSCILLATORY CIRCUIT AND RESPONSIVE TO CHANGES IN THE FREQUENCY OF OSCILLATION OF SAID OSCILLATORY CIRCUIT TO OPPOSE AND COMPENSATE FORCHANGES IN AMPLITUDE OF SAUCH OSCILLATIONS DUE TO CHANGES IN RESISTIVITY OF THE BASE METAL AND TO ASSIST CHANGES IN AMPLITUDE OF SUCH OSCILLATIONS DUE TO CHANGES IN THE SPACING OF SAID MOVABLE INDUCTOR FROM SAID BASE METAL AND A TEMPERATURE RESPONSIVE ELEMENT ELECTRICALLY CONNECTED BETWEEN THE CATHODE CIRCUIT OF SAID CATHODE FOLLOWER TUBE AND SAID PARALLEL TUNED CIRCUIT TO MODIFY THE FEED-BACK IN SAID LOOP IN THE SENSE TO OPPOSE ANY VARIATION IN THE Q OF SAID PARALLEL TUNED CIRCUIT DUE TO VARIATIONS IN AMBIENT TEMPERATURE.