NDT Eddy Current (ET symbol) is to create, in electrical conductive materials currents induced by a sensor.

These induced currents called eddy currents flow locally in the material and a distribution and a distribution which depend on the electro- magnetic excitation field, the geometry and electrical conductivity characteristics and magnetic permeability of the examined part. In the presence of an abnormality in controlled room, eddy currents are disrupted, resulting in a change in the apparent impedance of the sensor.

It is the analysis of the change in impedance which is translated into amplitude and phase on a screen as curves called "Lissajous" when doing research defects. Eddy currents can also be used to measure the conductivity of the material, measuring the thickness of non-conductive coatings on a conductive substrate. In this case, using simpler devices with a dedicated display.

This is a method of local surface defects detection.