Abstract: The study of the distribution characteristics of the surface charge density in the constant current field plays an important role to understand the constant current field. The mirror principle is used to derive the formula of charge density distribution in two kinds of infinite interfaces （one is the interface of two-layer conductive medium ; another is between the conductive medium and the air） while both emission and return electrode are placed in a conductive medium. By using the software of the COMSOL, the surface charge density under the same circumstance but limited medium is calculated. The results show that on the interface between the medium and air in the homoge- nous medium, one side of the emission electrode is positive charge and the other side is negative, without charge in centre. Whether the charge on the interface between different conductive media is positive or negative is decided by the difference of two side resistivities. The value of the charge density on the interface is inversely proportional to conductivity. In the limited medium, the numerical calculation reveals quantitatively the charge distribution features in corner, edge and intersecting line on the surface. They depend on the position of the emission electrode, the return electrode and the surface geometrical structure.

Key words: constant current field, charge distribution, interface charge, surface charge