الفيزياء العامة الجزء الثاني#موقع الفيزياء.كوم

Appendices
XEROGRAPHY (Photocopier)
XEROGRAPHY, electrostatic dry-printing process for the reproduction of
images or documents, widely employed in commerce and industry in
copying machines
The process was invented by the American printer Chester F. Carlson
(1906–68) in 1937 and first commercially developed in 1950. It makes use
of the principle of photoconductivity, that is, that certain substances resist
passage of an electric current except when struck by light. Silicon,
germanium, and selenium are poor conductors of electricity, but when light
energy is absorbed by some of their electrons, the electrons are able to pass
from one atom to another, thus allowing a current to flow when a voltage is
applied. When the light is removed, their conductivity again becomes low.
Xerography employs a photoconductive insulating layer, such as selenium,
on an aluminum or other conductive metal support.
The layer is charged electrostatically, either with
positive or negative ions, the polarity of the charge
depending on the type of photoconductive
insulating layer selected. When the plate is
exposed, in a camera or photographic machine,
those areas of the coating subjected to light lose a
varying portion of the charge, depending upon the
intensity of the illumination. Thus, the variation of
the amount of charge retained on the coated metal
plate is established as an electrical or electrostatic
pattern of the image. The image is rendered visible
by sprinkling over the exposed plate a special,
charged powder, which carries an opposite charge
to the initial charge applied to the plate and insulating layer. The powder
adheres to those areas that have retained their charge. The print is obtained
by covering the plate with paper, then applying a charge over the back of
the paper of the same polarity as the initial charge applied to the
photoconductive insulating layer. In this way the opposite charged powders
are transferred to the paper surface. The powder image is then fused onto
the paper by exposure to solvent vapors or heat to make the image
permanent.
The entire xerographic process can be carried out, in high-speed
mechanized equipment, in less than 5 seconds, and it is comparatively
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