Selenium is the element used as the basis of
photocopy or
xerox machines (based on
xerography, from Greek for 'dry writing'). Such machines take advantage of selenium's ability to conduct an electrical current only when illuminated.
Inside the machine, a drum is coated in a film of selenium and electrostatically charged. When a document is exposed, an image of it is projected onto the selenium coated drum, producing a map of high and low positive charge corresponging to the bright and dark areas on the paper. Negatively charged dry ink particles are attracted to the regions of high enough positive charge, and from there they are transferred electrostatically to the print paper as it is rolled over the drum. The ink particles are fused with heat or by application of a solvent, and the pattern of charge on the drum is dissippated by exposure to strong light, leaving the selenium film ready to record the next page.