Combined leakage field and eddy current detection system

Claim 1

IN A SYSTEM FOR DETECTING EITHER SURFACE TYPE DEFECTS OR SUB-SURFACE TYPE DEFECTS IN A TEST PIECE OF FERROUS MATERIAL HAVING A PERMEABILITY CURVE SUCH THAT WITH AN APLIED FIELD INCREASING FROM ZERO TOWARD A SATURATING VALUE THE ABSOLUTE VALUE OF THE FIRST DERIVATIVE OF THE PERMEABILITY CURVE INCREASES TO A PEAK VALUE AT A FIRST FIELD STRENGTH VALUE, THEN DECREASES TO ZERO AT A SECOND FIELD STRENGTH VALUE, THEN INCREASES TO ANOTHER PEAK VALUE AT A THIRD FIELD STRENGTH VALUE AND FINALLY DECREASES TO ZERO AT A FOURTH RELATIVELY HIGH SATURATING FIELD STRENGTH VALUE, TEST COIL MEANS DISPOSED IN PROXIMITY TO A SURFACE OF SAID PIECE FOR INDUCING AN A.C. FIELD IN SAID PORTION AND INDUCING EDDY CURRENTS THEREIN AND FOR DEVELOPING A SIGNAL IN RESPONSE TO SAID FIELD AND SAID EDDY CURRENTS, AND MEANS FOR APPLYING A UNIDIRECTIONAL BIASING FIELD IN SAID TEST PIECE HAVING A VALUE APPROXIMATELY EQUAL TO ONE OF SAID SECOND AND THIRD FIELD STRENGTH VALUES FOR PRODUCING A MAXIMUM CHANGE IN SAID SIGNAL IN RESPONSE TO ONE OF SAID TYPES OF DEFECTS WHILE PRODUCING A MINIMUM CHANGE IN SAID SIGNAL IN RESPONSE TO THE OTHER OF SAID TYPES OF DEFECTS.