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Photocopying is a process which makes paper copies of documents and other visual images quickly and cheaply. Most current photocopiers use a technology called xerography, a dry process using heat. more...
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(Copiers can also use other output technologies such as ink jet, but xerography is standard for office copying.)
Xerographic office photocopying was introduced by Xerox in the 1960s, and over the following 20 years it gradually replaced copies made by Verifax, Photostat, carbon paper, mimeograph machines and other duplicating machines. The prevalence of its use is one of the factors that prevented the development of the paperless office heralded early in the digital revolution.
Photocopying is widely used in business, education, and government. There have been many predictions that photocopiers will eventually become moot as information workers continue to increase their digital document creation and distribution, and rely less on distributing actual pieces of paper. However, photocopiers are undeniably more convenient than computers for the very common task of creating a copy of a piece of paper.
How a photocopier works (using xerography)
The surface of a cylindrical drum is given an electrostatic charge by either a high voltage wire called a corona wire or a charge roller. The drum is coated with a photoconductive semiconductor material, such as selenium.;
The light reflected from the scanned original document is beamed in a narrow band onto the surface of the drum. Only the white areas of the original document reflect light. The reflected light then hits the drum, which is photoconductive. This means that where light hits it the charge is conducted away to a ground.;
As a result, the white areas of the picture are now neutral, and the black areas remain positively charged, yielding a latent electrical image on the surface of the drum.;
The toner is negatively charged. When it is applied to the drum to develop the image, it is attracted and sticks to the areas that are positively charged, just as paper sticks to a toy balloon with a static charge.;
The toner image is then transferred from the drum onto a positively charged piece of paper.;
The drum is then wiped clean and completely discharged by light, before beginning the process again.;
The toner is a dry powder. If copies were to exit the photocopier covered in dry toner it would easily brush off. Toner usually contains a styrene or polyester resin, and with the application of heat and pressure it melts and bonds (fuses) to the paper.;
Invention
In 1937 Bulgarian physicist Georgi Nadjakov found that when placed into electric field and exposed to light, some dielectrics acquire permanent electric polarization at the exposed areas. That polarization persists in the dark and is destroyed in light. Chester Carlson, the inventor of photocopying, was originally a patent attorney and part time researcher and inventor. His job at the patent office in New York required him to make a large number of copies of important papers. Carlson, who was arthritic, found this a painful and tedious process. This prompted him to conduct experiments with photoconductivity. Carlson experimented with "electrophotography" in his kitchen and in 1938, applied for a patent for the process. He made the first "photocopy" using a zinc plate covered with sulfur. The words "10-22-38 Astoria" were written on a microscope slide, which was placed on top of more sulfur and under a bright light. After the slide was removed, a mirror image of the words remained. Carlson tried to sell his invention to some companies, but because the process was still underdeveloped he failed. At the time multiple copies were made using carbon paper or duplicating machines, and people did not feel the need for an electronic machine. Between 1939 and 1944, Carlson was turned down by over 20 companies, including IBM and GE, neither of which believed there was a significant market for copiers.
Read more at Wikipedia.org
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