Abstract: This paper explains briefly the introduction of the proportional coefficientα=σs /σl (σs is areal density of magnetic charges on the bottom surface of a workpiece or on the lateral walls of a slot in the stable condition of magnetization, σl is linear density of magnetic charges along the base apex lines of a workpiece or along the slot edges). And its dimension as well as unit are also given. The effect using this new concept was that the spacial distribution curves of magnetic leakage field yielded by a rectangular slot on the workpiece surface obtained by Dr. F. Forsters experiment in 1985 was expounded quantitatively for the first time.