The characteristics of a contaminated point-plane electrode have been studied in the range from the onset of back corona to the electrical breakdown in the contaminant. The distribution of surface charge accumulating on the contaminant is found to follow the normal distribution curve. The exponential relationship between the current and voltage in the contaminant indicates that the main contribution to the total current is from surface conduction at low electric fields and emission current at high fields. As the separation distance between adjacent volumes of homogeneous contaminant increases, the induced field which is caused by the constriction of current flow may be sufficiently large to activate in the contaminant a local gas discharge known as back corona. When the electric field in the proximity of the contaminant where back corona occurs is known, the critical resistivity of the contaminant can be found.