Eddy current testing system having concentric coils, one being movable for balancing

Claim 1

In an eddy current testing system for measuring characteristics of a specimen, a test coil unit arranged to receive the specimen therein, said test coil unit comprising coil form means including first, second and third annular forms each having an annular slot in the outside surface thereof, said forms being concentric and in radial alignment with said first form being fitted inside said third form and with said third form being fitted inside said second form, first and second windings in said annular slots of said first and second forms and having approximately the same axial and radial dimensions, wire sizes and members of turns, a third winding in said annular slot of said third form, an AC source connected to said third winding to develop AC voltages in said first and second windings, one of said first and second forms being adjustably positionable in an axial direction relative to the other to a position such as to equalize the instantaneous voltages developed in said first and second windings with no specimen disposed in said coil unit, and locking means for fixing said one of said forms in said position, and measuring means directly connected to said first and second windings to measure instantaneous differences in the voltages developed across said windings.