Abstract: Measurements of complex permittivity using transient techniques and Fourier transforms require computation which can be lengthy. In the invention, a synthesizer generates a succession of waveforms each containing a small number of harmonically related frequency components. These waveforms are applied to a capacitive potentiometer formed by a reference capacitor and a sample head. The voltages across the potentiometer and the sample head are passed by way of an analog-to-digital converter to a microprocessor which calculates Fourier transforms and indicates permittivity versus frequency. By limiting the frequency components, the computation time is reduced and a quick scan to find spectral regions of interest is possible. These regions can then be examined at higher resolution by changing a clock frequency which determines the generation of the frequency components.

Abstract: A circuit for providing a controlled resistance, the circuit comprising a pair of terminals for input of an excitation current, variable resistance means connected in the current path between the terminals, means for deriving a first signal representing the value of the voltage appearing across the terminals, means for deriving a second signal representing the value of the excitation current, means for generating a third signal representing a selected value of resistance to be provided between the pair of terminals, means for comparing the ratio of the first and second signals with the value of the third signal to derive a difference signal, and means for applying the difference signal to cause the variable resistance means to change in value such that said ratio tends towards exact equality with the selected value of resistance.