Frequency Properties of an Air-Cored Coil and Application to Conductivity Measurement

This study investigated the frequency properties of an air-cored coil and applied it to inspect conductivity change. Theoretical analysis and experimental study showed that what we measured is not the coil's own resistance and inductance but the equivalent ones which change with frequency. Case studies on conductivity measurement using a coil probe showed that we can greatly enhance measurement sensitivity by making the best of the coil's frequency properties. By utilizing the coil at frequencies that the coil's resistance and inductance are `frequency-invariant', we were able to detect conductivity change with high sensitivity on the basis of `same product of conductivity and frequency, ωσ(product of angular frequency and conductivity), same impedance' principle. The study also showed that we can use the coil's resonant properties, e.g., resonant frequency, resistance at resonant, to sense conductivity change, regardless of change on thickness. A slight variation of conductivity could be perceived by fine frequency adjustment. The results strongly suggest using a coil in accordance with its frequency characteristics.