How is static elctricity used in xerography?

Answer 1

Xerography, developed by patent lawyer Chester Carlson, depends upon the property of the metal selenium to hold a static charge whilst in the dark but lose it when exposed to light.

Originally it used a sheet of aluminium coated with selenium to act in much the same way as a sheet of light sensitive film. The selenium was charged with static electricity form a corona discharge and kept in the dark. The sheet was then exposed through a camera to a brightly lit object, usually a sheet of white paper with black printing on it.

Where the light from the bright areas of the image impinged upon the selenium layer the static charge discharged into the aluminium backing sheet, leaving an image, in static electricity, on the surface of the selenium.

This image was then developed by cascading it with fine, heat or vapour fusible, plastic powder (carried on small plastic coated glass beads) which was attracted to and adhered to the electrostatic image.

A sheet of paper or card was then laid over the image and the powder was transferred to it, once again using static electricity to cause the transfer.

Then, using either heat or the solvent vapour of trichlorethylene, the image was fused onto the paper or card.

Initially the process was carried out manually and used mainly to image paper 'plates' for the offset litho printing process but in about 1961, a faster rotary process was developed, using a selenium coated aluminium drum. This equipment (called incidentally the Xerox 'nine-fourteen') was the first true plain paper copier and the start of the whole xerographic copying and printing industry. Xerography was also used for some years together with X-ray equipment for engineering X-ray examination and for mamographic X-rays. As a matter of interest the 'nine-fourteen' was larger than a big desk and was so heavy it took 3 or four men to move it!