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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