The effects of stray capacitance on the Kelvin method for measuring contact potential difference

A simple theory is given of the problem of stray capacitance in the Kelvin method and experimental results are presented which show the importance of its effects. These may cause differences of several tenths of a volt between the apparent contact potential difference and the true one. The magnitude of its effects is shown by these experiments to depend linearly on the following three factors: the cpd between the vibrating electrode and the walls of the vacuum chamber, the separation of the Kelvin electrodes and the amplitude of vibration. The simple theoretical approach correctly predicts the linear variation with the first of these factors. In order to reduce these effects to a minimum it is recommended that the stationary electrode should be connected to the input terminal of the detector, while the vibrating electrode and the walls of the vacuum chamber should both be earthed, and the vibrating electrode should preferably act as the reference surface.