Potentials in D-C corona fields

A probe method has been devised for measuring potentials in d-c corona fields which are uniform for a short distance in one direction. In unipolar corona, a probe supported by perfect insulators assumes a potential sufficiently above the space so that no more ions can reach the probe. For a cylindrical probe with its axis along an equipotential, this excess or “error potential” is proportional to the diameter of the cylinder. To minimize this error a fine wire is used as the probe. A frame is designed which both supports the wire and shields it from fields other than the field being measured. The fine wire is electrically connected to an electrostatic voltmeter, and the frame to a source of variable potential. A potential is measured by slowly increasing the potential of the frame and recording both frame and wire potential. Under proper conditions, the space potential is the point where wire and frame potentials are equal.