| 摘要 |
PURPOSE:To accurately detect minute surface cracks, by moving a detection coil under a condition where the coil is contacted with or brought nearer to an object to be measured, measuring the voltage across both ends of the coil, and comparing the measured voltage with a reference voltage. CONSTITUTION:An electric current of an intermediate frequency is sent to a detection coil 1 from an oscillator 2. Then a voltage is produced across both ends of the coil 1 by moving the coil 1 under a condition where the coil is contacted with or brought nearer to a ferromagnetic material. The voltage across both ends of the coil 1 is inputted to a comparison amplifier 5 through an amplifier 6 and rectifier 7. Moreover, a set reference voltage 8 is also inputted to the comparison amplifier 5 and the differential voltage between both voltages is detected with a voltmeter 10. When the detection coil 1 crosses over a crack, an eddy current is reduced and the impedance of the coil 1 is increased and, as a result, a change in the voltage across both ends of the coil 1 is displayed in the voltmeter 10. Since a change in the voltage across the coil 1 is compared with a reference value, even a minute crack can surely be detected.
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