Assessment and improvement of electric-field measurements using a bias probe with particular reference to twin-point-plane geometry

A commonly used method of measuring current density and electric field at the surface of an electrode is by the use of a bias probe which is inserted into the contours of the electrode. Values of the electric field can be found by applying a bias voltage to the instrument and measuring the ratio of received currents, with and without this bias voltage in operation. The field strengths are then read off a previously obtained calibration curve. These curves are all obtained under the assumption that the current densities and electric fields to be measured are constant in the vicinity of the instrument. Such direct use of the calibration curves in regions with significant spatial field variations are here shown to give spurious results, for example there are large oscillations in field measurements as a probe traverses an abrupt change in current. The anomalies are well exhibited by measurements on a twin-point-plane corona discharge in air, and the behaviour has been explained and quantified by using a simple model. An improved measurement procedure has been developed for such situations. This has been expedited by fitting an analytic curve to the calibration and then programming a method which uses adjacent readings to predict the field strength. The anomalous readings have been shown to be false, and significant overestimates of the field strength can be avoided by following the procedures described in the paper.