Frequency Dependence of Electrode Surface Effects in Parallel-Plate Capacitors

Electrode surface effects in parallel-plate capacitors lead to a variation in capacitance and conductance with frequency. A method is presented for the determination of absolute changes in capacitance and absolute values of conductance, as functions of frequency. The method involves the use of two variable parallel-plate capacitors, and requires the measurement of relative capacitance and conductance values over a range of frequencies and at a minimum of two different electrode spacings. A capacitor cell, incorporating two variable capacitor sections is described. The cell is designed for use in the investigation of the frequency characteristics of a range of electrode surfaces. Two current transformer ratio-arm bridges are used to perform the relative admittance measurements. The bridges are compensated for capacitance loading, and together cover the frequency range 11 Hz-52 kHz. Measurement precision is a few parts in 108 for each admittance component. Details of the bridges and measurement techniques are discussed and corrections for mechanical resonance effects and lead impedances are considered. Results are given for rhodium plated and plain brass electrodes under vacuum. Over the full frequency range the fractional capacitance change for rhodium, when referred to a 1-mm electrode spacing, is <4 × 10-7. Tan δ is < 1.5 × 10-7. Oil films and organic contaminants on electrode surfaces are thought to be the most likely sources of frequency dependence.