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Edd y current testing uses induced currents in the test piece to reveal flaws. An alternating current in a nearby probe coil creates a changing magnetic field that induces eddy currents in the conductive material just beneath the surface. Flaws or changes in material properties disrupt these currents, which changes the coil’s impedance or the signal detected by the instrument, signaling a defect. This is an electromagnetic nondestructive test, well suited for detecting near-surface flaws in conductive metals. Ultrasonic testing relies on sound waves, radiographic testing uses X-rays or gamma rays to image interiors, and the broader acoustic category isn’t about induced currents in the piece. So the described mechanism points to electromagnetic (eddy current) testing.