In Eddy Current Testing, a coil carrying an AC current is placed close to the specimen surface, or around the specimen. The current in the coil generates circulating eddy currents in the specimen close to the surface and these, in turn, affect the current in the coil by mutual induction. Flaws and material variations in the specimen affect the strength of the eddy currents.

The presence of flaws etc. is therefore measured by electrical changes in the exciting coil. Both voltage and phase changes can be measured, but some simpler instruments measure only the voltage changes.

Items related to this area include (among others): portable testers, inline testers & hand rotors.