the first step the reference is first made to fig3 which shows an equivalent circuit corresponding to the simultaneous charge transfer mechanism shown in fig2 in this figure u03c7p u03c7a and u03c7d respectively represent the air gap equivalent thicknesses in microns of photoconductive layer 13 the air gap between the virtually contacting photosensitive and dielectric members and of the dielectric member itself here the air gap equivalent thicknesses are obtained by assuming that photoconductive layer 13 the air gap and dielectric layer 21 respectively are dielectrics and the thicknesses of these dielectrics are each divided by their relative dielectric constants additionally the electrostatic capacity in units of pf cm 2 of the respective dielectrics are determined from c 885 u03c7 also vap in fig3 represents the applied voltage here vb u03c7a is the air breakdown discharge initiating voltage in accordance with paschen law and that with u03c7a it is determined from 312 6 2 u03c7a as to u03b5o it represents the dielectric constant qt the amount of charges transferred onto the dielectric member q the amount of charges induced on the photoconductive layer air gap and dielectric member and vao the potential in the air gap prior to the application of voltage accordingly the surface potential v.