Speed sensor for a rotating member

A speed sensor for use in measuring the speed of rotation of a rotationally salient rotating member The speed sensor comprises an electrode and a sensor circuit. The sensor circuit comprises a constant voltage source for supplying a voltage to the electrode to generate an electric field in a dielectric medium. A current detector detects current flow between the constant voltage source and the electrode due to perturbation of the electric field by passage of at least one salient feature of the rotating member through the electric field as the rotating member rotates. The current detector outputs a first signal modulated at a frequency corresponding to the frequency of perturbation of the electric field. The first signal is amplified to produce an amplified signal modulated at a frequency corresponding to the frequency of perturbation of the electric field. The electrode voltage, the amplifier gain and the electrode position relative to the rotating member are selected such that modulation of the second signal is predominantly caused by perturbation of the electric field within the dielectric medium by the creation and/or movement of ions within the field.