An apparatus for developing latent electrostatic images in an electrophotographic copier are disclosed, the developer including an image carrier having a conductive backing layer and a photoconductive layer, a magnetic brush developer, a voltage source connected with the magnetic brush developer, and a high-ohmic resistance connecting the backing layer with ground, the voltage source and high-ohmic resistance establishing a potential difference between the magnetic brush developer and the backing layer to prevent a voltage breakdown at the image carrier which may result in undesirable streaking and fogging of an electrostatic copy. The high-ohmic resistance may be adjusted in accordance with the amount of current generated in the developer mixture whereby the flow of current from the developer through the photoconductive layer to the backing layer is impeded to prevent a voltage breakdown. In a first embodiment, the image carrier is a planar printing plate. In a second embodiment, the image carrier is a cylindrical drum having the photoconductive layer arranged on its outer peripheral surface, with charge and transfer coronas arranged adjacent the photoconductive surface, and the embodiment includes a semiconductor unit connected in parallel with the high-ohmic resistance.