The eddy currents are currents that appear in a conductive material is varied when the magnetic flux in the vicinity. They are a consequence of the law of Lenz-Faraday.
As part of the CND is the magnetic induction B which is varied by injection into a coil of an alternating current in different frequencies. This coil plays the role of transmitter and receiver sometimes, the flux variation in the material creates induced currents: the eddy current (FC). In the presence of a defect, their movement is impaired by the electrical conductivity changes due to the geometry of the defect. This disruption causes a change in the magnetic field created by the eddy currents that can be detected through the coil (inductive flow meter sensor types). The technique involves using a probe having a coil and measure across a bridge differential voltage generated by the change in impedance of the exciting coil / receiver.
Non-destructive testing by eddy currents can be automated which makes it quick check; and convenient especially if the control environment is potentially dangerous.In the presence of a complex default, additional ultrasound examination can be performed to better characterize the anomaly.
Source : Wikipedia