10/2/2023 0 Comments Photocopy effectA toner is a positively charged material that is added to the photoconducting surface. The next step in copying involves the addition of a toner to the photoconducting surface. Similarly, areas of gray in the original document are also matched on the photoconducting surface because greater or lesser amounts of light are reflected off the document, causing greater or lesser loss of negative charge on the photoconducting surface. A letter a, for example, on the original document becomes an a-shaped region of negative electrical charge on the photoconducting surface. Light regions on the original document (such as blank spaces) do reflect light to the photoconducting surface, causing the loss of negative charge in these regions. Therefore, those portions of the photoconducting surface retain their negative charge. Dark regions on the original document (such as printed letters) do not reflect any light to the photo-conducting surface. Notice the way the image on the original document is transferred to the photoconducting surface. When light strikes the photoconducting surface, it erases the negative charges there. The light reflected off that document is then reflected off a series of mirrors until it reaches the negatively charged photoconducting surface. In another part of the machine, the original document to be copied is exposed to light. The negative ions thus produced are repelled by the negatively charged wire and attach themselves to the photoconducting surface. When the wire is charged negatively, a strong electrical field is produced that causes ionization of air molecules in the vicinity of the wire. The charged placed on the photoconducting surface is usually obtained from a corona bar, a thin wire that runs just above the surface of the photoconducting surface. The surface could be a selenium-coated drum or an endless moving belt mounted on rollers, for example. The core of such machines is a photoconducting surface to which is added a negative charge of about 600 volts. Many different models of xerographic copying machines are available today, but they all operate on some common principles. Among these other forms of photocopying are thermography, diazo processes, and electrostatic copying. However, a number of other forms of photocopying pre-dated the Carlson invention and are still used for special applications. Indeed, the name of the company founded to develop Carlson ’s invention, Xerox Corporation, has become synonymous with the process of photocopying. Today the most widely used form of photocopying is xerography (dry writing), which was invented by Carlson. Nadjakov ’s work with the photoelectric effect led to the invention of the photocopier in the 1930s by American physicist and inventor Chester Carlson (1906 –1968), who was also a New York patent attorney. Bulgarian physicist Georgi Nadjakov (1896 – 1981) discovered that some dielectrics (materials that are unable to directly conduct electric current) became permanently polarized when placed into an electric field and exposed to light. Photocopying is the process by which light is used to make copies of book pages and other paper documents.
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