Principles of eddy-current distance gauges

The field theory of an a.c. loop over a metal surface is reviewed and expressions are derived which demonstrate the possibility of a method of distance gauging, in which the effects of conductivity and permeability of the metal can be compensated. The residual interaction then approaches that of a perfect image of the loop reflected in the surface. Some experimental gauges are described and their measured performance is presented, confirming the validity of the method.