Multi-frequency testing method and apparatus for selectively detecting flaws at different depths

Claim 1

IN A MULTI-FREQUENCY EDDY-CURRENT TESTING SYSTEM, AN OSCILLATOR CIRCUIT TUNABLE OVER A WIDE RANGE OF RESONANT FREQUENCIES, INCLUDING A VACUUM TUBE OSCILLATOR OF THE TYPE DRAWING SUBSTANTIALLY NO GRID CURRENT, A TANK CIRCUIT HAVING ADJUSTABLE CAPACITANCE FOR TUNING THE OSCILLATOR CIRCUIT OVER A WIDE RANGE OF RESONANT FREQUENCIES, A DETECTOR COIL ADAPTED TO BE POSITIONED IN INDUCTIVE RELATION TO A TEST PIECE, A TRANSFORMER HAVING A PRIMARY AND A SECONDARY COUPLED TOGETHER WITH SUBSTANTIALLY UNITY COUPLING, SAID PRIMARY BEING CONNECTED IN SAID TANK CIRCUIT, AN A.C. FEEDBACK CONNECTION FROM AN ELECTRODE OF THE VACUUM TUBE IN SAID OSCILLATOR TO A POINT OF INTERMEDIATE IMPEDANCE BETWEEN THE TERMINALS OF SAID PRIMARY, AND CONNECTIONS FROM THE TERMINALS OF SAID SECONDARY TO THE TERMINALS OF SAID DETECTOR COIL, SAID DETECTOR COIL AND THE REACTANCE OF SAID CONNECTIONS RESONATING AT A FREQUENCY HIGHER THAN THE HIGHEST FREQUENCY TO WHICH SAID OSCILLATOR CIRCUIT IS TUNABLE IN NORMAL USE IN SAID EDDY-CURRENT TESTING, AND METER MEANS COUPLED TO SAID OSCILLATOR CIRCUIT FOR MEASURING THE AMPLITUDE OF THE OUTPUT VOLTAGE THEREOF OVER SAID RANGE OF RESONANT FREQUENCIES.