xerographyArticle Free Pass xerography, Image-forming process
that relies on a photoconductive substance whose electrical
resistance decreases when light falls on it. Xerography is the
basis of the most widely used document-copying machines (see
photocopier). The process was invented in the 1930s by U.S.
physicist Chester F. Carlson (1906–1968) and developed in the
1940s and ’50s by Xerox Corp. (then called Haloid). Light
passing through or reflected from a document reaches a selenium-
coated drum surface onto which negatively charged particles of
ink (toner) are sprayed, forming an image of the document on the
drum. As a sheet of paper is passed close to the drum, a
positive electric charge under the sheet attracts the negatively
charged ink particles, transferring the image to the copy paper.
Heat briefly applied fuses the ink particles to the paper. The
first commercially successful xerographic copier was introduced
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