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Eddy Current Principle

The measuring systems use the principle of the eddy current effect. Therefore, a high-frequency magnetic field is generated and the sample is placed in the magnetic field.
The conductive material acts on the resonant circuit, such as a resistive load and thus leads to a change in the power consumption in the oscillator circuit.
Now, the difference in power is determined and according to findings by Miller (GL Miller, DAH Robinson, and JD Wiley, "Contactless measurement of semiconductor conductivity by adio frequency-free-carrier power absorption", Rev. Sci. Instrum, Vol . 47 (1976) the difference is proportional to the conductance of the material. With sufficient thickness of the material, the value is the direct read out of the specific conductance (reciprocal, the resistivity) of the material. In case the material is thin compared to the magnetic field, this value converts to the sheet conductance of the material.

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