Variable frequency eddy current test device with variable means for maintaining the apparent impedance of the probe constant at all frequencies

Claim 1

IN A MULTI-FREQUENCY APPARATUS FOR USE IN EDDY CURRENT TESTING, A SOURCE OF DRIVE SIGNALS HAVING A FIRST FREQUENCY SELECTED WITHIN A RANGE OF TEST FREQUENCIES, MEANS FOR CHANGING THE FREQUENCY OF SAID DRIVE SIGNALS TO A SECOND FREQUENCY WITHIN SAID RANGE, A FIXED INDUCTANCE PROBE ARRANGED AND ADAPTED TO BE EXCITED BY SAID DRIVE SIGNAL TO PROVIDE AN OUTPUT, SAID OUTPUT REPRESENTING CHARACTERISTICS OF A TEST SPECIMEN WHEN SAID PROBE IS LOADED BY SAID SPECIMEN, MEANS PROVIDING A STANDARD IMPENDANCE OUTPUT FOR COMPARISON WITH SAID PROBE OUTPUT WHEN SAID PROBE IS LOADED BY SAID SPECIMEN, VARIABLE TRANSFORMER MEANS CONNECTED BETWEEN SAID SOURCE OF DRIVE SIGNALS AND SAID PROBE, CONTROL MEANS COUPLED TO SAID TRANSFORMER MEANS TO VARY THE TRANSFORMER MEANS FOR MAINTAINING AN APPARENT IMPEDANCE OF SAID PROBE CONSTANT AT SAID FIRST AND SAID SECOND FREQUENCIES IN THE ABSENCE OF SAID SPECIMEN, MEANS FOR COMPARING SAID STANDARD IMPEDANCE OUTPUT WITH SAID PROBE OUTPUT TO PROVIDE A DIFFERENCE SIGNAL WHEN SAID PROBE IS LOADED BY SAID SPECIMEN, AND MEANS RESPONSIVE TO SAID DIFFERENCE SIGNAL FOR INDICATING CHARACTERISTICS OF SAID SPECIMEN.