a developing unit for use with an electrophotographic copying apparatus of the type comprising a developing electrode and at least one corona discharger includes a conductive member arranged within a shield casing of the discharger for obtaining a voltage to be applied to the developing electrode as a bias voltage thereto . in the case where an a . c . discharger for charge elimination is being used in addition to the primary discharger , conductive members are incorporated with respective dischargers and the outputs from the conductive members are superimposed on each other and applied to the developing electrode . in this instance , the output of the conductive member associated with the a . c . discharger is continuously applied to the developing electrode after the de - energization of the primary discharger , i . e ., the completion of developing an electrostatic latent image to thereby prevent the deposition of toner on the developing electrode .