Photocopying

Many different models of xerographic copying machines are available today, but they all operate on some common principles. The core of such machines is a photoconducting surface to which is added a negative charge of about 600 volts. The surface could be a selenium-coated drum or an endless moving belt mounted on rollers, for example. The charged placed on the photo-conducting surface is usually obtained from a corona bar, a thin wire that runs just above the surface of the photoconducting surface. When the wire is charged negatively, a strong electrical field is produced which causes ionization of air molecules in the vicinity of the wire. The negative ions thus produced are repelled by the negatively charged wire and attach themselves to the photo-conducting surface.

In another part of the machine, the original document to be copied is exposed to light. The light reflected off that document is then reflected off a series of mirrors until it reaches the negatively-charged photoconducting surface. When light strikes the photoconducting surface, it erases the negative charges there.

Notice the way the image on the original document is transferred to the photoconducting surface. Dark regions on the original document (such as printed letters) do not reflect any light to the photoconducting surface. Therefore, those portions of the photoconducting surface retain their negative charge. Light regions on the original document (such as blank spaces) do reflect light to the photo-conducting surface, causing the loss of negative charge in these regions. A letter "a" on the original document becomes an a-shaped region of negative electrical charge on 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.