Eddy Current Inspection
In eddy current inspection, the eddy currents create their own electromagnetic field, which can be sensed either through the effects of the field on the primary exciting coil or by means of an independent sensor.  In nonferromagnetic materials, the secondary electromagnetic field is derived exclusively from eddy currents.  However, with ferromagnetic materials, additional magnetic effects occur that are usually of sufficient magnitude to overshadow the field effects caused by the induced eddy currents.  Although undesirable, these additional magnetic effects result from the magnetic permeability of the material being inspected and can normally be eliminated by magnetizing the  material to saturation in a static (direct current) magnetic field.  When the permeability effect is not eliminated, the inspection method is more correctly categorized as electromagnetic or magnetoinductive inspection.

Remote-Field Eddy Current (RFEC) inspection is a nondestructive examination technique suitable for the examination of conduction tubular goods using a probe from the inner surface.  Because of the RFEC effect, the technique provides what is, in effect, a through-wall examination using only the interior probe.  Although the technique is applicable to any conducting tubular material, it has been primarily applied to ferromagnetics (steels).