Measurement of eddy-current conductivity

The conductivity of the surface layers of a metal is measured industrially by eddy-current conductivity meters which need to be calibrated. The method described here allows the resistivity of an annular specimen, as measured by the eddy-current losses at frequencies in the range 10¿¿100 kHz, to be measured in units traceable to national standards. The apparatus is based on a Heydweiller bridge which measures the mutual inductance and mutual resistance of two toroidal windings on the annular specimen. Corrections are needed for the effects of self capacitance and the capacitance between windings. The overall uncertainty in the measurement is from about 0.4% at 10 kHz to 0.6% at 100 kHz, and it could not easily be improved. This is adequate for present purposes.