Disclosed is an improved apparatus for applying a uniform electrostatic charge to a predetermined portion of an electrophotographic film. The apparatus includes a corona generation source and a mask framing the predetermined portion of the film. The improvement consists of an electrically conductive surface surrounding the frame opening of the mask and spaced from the surface of the film and a capacitor interconnecting the electrically conductive surface with ground. The electrically conductive surface on the mask develops a voltage close to that of the surface potential of the film during corona charging so that very little charge field discontinuity exists between the mask and the film, thereby permitting uniform electrostatic charging of the film up to the edges of the portion being charged.

A method of controlling the area developed by a corona discharge which consists in providing a shield around the corona point and controlling the effect to the shield to have the focussing effect on the corona preferably by controlling the position of the electrical charge or the electrical resistivity of the shield.

A compound surface-charging electrode (6) with a metal housing (11). The housing accommodates two or more unit electrodes and upstream resistances (1) that can be connected to a source (U) of high voltage. The unit electrodes have prongs (3) that project out of the housing. The housing is open enough to allow them to do so. Each prong can be covered up by an electrically conductive electrode hatch (4). The electrode hatch is electrically conducting and connected to the housing. The housing and the electrode hatch are connected over to one terminal of a resistor, the other terminal of which can be grounded.

I provide charging apparatus for an electrostatic copying machine of the type in which a photoconductive material is charged electrostatically and exposed to a radiation pattern to form a latent image which is developed by the application of suitable ink, ink particles or colored toner. The charge retained by the material is a function of two major variables, the strength of the field reaching the photoconductive material and the duration of its exposure to the charging field. In my apparatus the charge is emitted by the charger at or near saturation level to produce a strong field. The area of the material exposed to this field at any time is limited in proportion to the field strength by shielding the material from the charge either physically, electrostatically or both.

In an electrophotographic copying apparatus, a device for preventing the charging device from charging selected marginal portions of a photoreceptor, to thereby prevent staining of these selected marginal portions when the charged part of the photoreceptor is exposed to the image of an original, and this image is later developed. Charging of the selected marginal portions of the photoreceptor is prevented by means of plates which are moved with respect to the charging device by manually moving indicators disposed along the length and width of the rest plate that supports an original for copying. Thus, only that portion of the photoreceptor which corresponds to the selected size of the original is charged, while the marginal portions of the photoreceptor which would surround the projected image of the original are not charged.