PURPOSE:To accurately obtain magnetic characteristic of an object to be tested by bringing a chamfer part of ring core which is made by a magnetic material and wound with primary and secondary coils into contact with the object to be tested, allowing AC current to flow in the primary coil, and measuring the inductance of the coil from the side of secondary coil. CONSTITUTION:The chamfer part 4 of ring core 1 is brought into contact closely with the magnetic material 9, then an alternative magnetic field is excited in the ring coil 1 by a sine wave come from an oscillator 5. The primary current at this time is measured by an ammeter 6. The secondary open-circuit voltage generated in the secondary coil 3 is measured by a voltmeter 7. Then from these measured values, an impedance relative permeability muZ of the magnetic material 9 is obtained by the formula. In this formula, V2 is the secondary open-circuit voltage, I1 is the primary current, and C is a constant decided by the characteristic of ring core and that of magnetic material (quality of material and thickness of plate). When a defective part is in existence on this material 9 and the chamfer 4 of ring coil is moved to come on this defective part, lines of magnetic force leak from the defective part and a magnetic resistance of ring core 1 is increased, then the defective part of magnetic material 9 is learned by the local decrease of secondary open-circuit voltage.
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