Abstract: The printing unit (1) operating with an electrostatic printing aid has an inductor device (7) for transmitting an electrostatic charge to an outer-shell layer (5) of a back-up cylinder (4). The inductor device is provided on one end face (14) of the back-up cylinder and engages over this end face in the manner of a lid. A continuous annular air gap (15) is located between the end face and the inductor device. Inductor electrodes (9) serve for transmitting the electrostatic charge to the outer-shell layer in a contactless manner and are arranged, concealed against outside access, on the inner face of the inductor device and are aligned by means of their electrode tips with an end-face edge (10) of the outer-shell layer (5). So that no impurities can penetrate into the air gap, it is sealed off or constantly scavenged by compressed air.