Electrostatic copying machines are known which comprise a rotatable cylinder which bears a surface layer of photoconductive material and which, as it rotates, brings successive parts of its surface first to a charging station, at which an electrostatic charge is imparted to the surface by a corona generator, then to an exposure station at which an image of a master to be duplicated is projected by an optical system to produce on the surface a charge pattern corresponding to the image areas of the master, then to a developing station at which a toner powder is applied to the image areas of the surface and finally to a transfer station at which the toner powder is transferred from the cylinder to a transfer sheet, which is thereafter subjected to treatment to fix to it the transferred toner powder. Such machines are expensive owing to the cost of the optical system and the corona generator and to the large amount of electrical power required for operation of the corona generator.
U.S. Pat. No. 2576047 discloses an electrostatic duplicator which uses a rotating cylinder carrying a metal master bearing an image of high electrical resistivity. It uses corona generators both to charge the image and to assist in transferring the toner to a continuous roll of transfer material. Each of the numerous corona generators used needs 6KV to operate it and corona generators are known to be difficult to maintain in a condition in which they apply an even charge to a master. This machine has not been put into use commercially.