This paper is concerned with theoretical and
experimental investigations of the onset voltage of
positive corona as influenced by coating the coronating
electrodes. Dielectric-coated hemispherically-capped
rod-plane gaps positioned in air are investigated. The
onset voltage is determined when a developed primary
avalanche triggers successor avalanches to sustain the
discharge process. Laboratory measurements of the
onset voltage on bare and coated electrodes were
carried out. The effects of varying field nonuniformity,
thickness and permittivity of coating material on the
onset voltage values were investigated. The results
show that coating the electrodes with a dielectric
material is effective in increasing the onset voltage of
corona. The calculated onset voltage values for coated
and bare electrodes agreed satisfactorily with those
measured experimentally within 10 %.