An experimental investigation has been carried out with sphere-to-plane geometrical electrodes for positive and negative corona discharge at atmospheric pressure and different inter-electrode separation. The experimental results have been compared particularly with those refer to point-to-plane and point-to-ring geometriesand a similar common feature has been provided. The current and voltage data was subjected to the relation of Townsend and Ferreira and showed one an upward curvature at low applied voltage (low field region) and then a linear behavior was found in high field region. This means that all acquired current-voltage data for both positive and negative coronas confirm Townsend relation at the inter-electrode distance used in this work. The extrapolated corona inception voltage was followed a non-linear tend with electrode gap distance S. The extrapolated potential Vo for positive corona was more sensitive on the distance S than that of negative corona. The dimensional constants A and K were found to be proportional inversely to the electrode gap distance by different power exponent. The ratios between the dimensional constants A and K for positive and negative corona approached a constant value at gap distance (S ≥ 2.5 cm).
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