After its introduction in the 1960s by Xerox, the photocopier machine became an extremely popular piece of equipment, virtually indispensable to many business and government offices. Replacing carbon paper, mimeograph, and other reproduction machines, photocopiers use the electric polarization of light sensitive material to produce accurate and inexpensive copies of paper documents. The advent of computers and electronic communication has modestly reduced the need for photocopies, but the due to their pervasiveness and ease of use, they remain widely utilized.
To produce photocopies of an original document, the photocopy machine first makes a temporary image, a sort of negative of the original. Inside the machine is cylinder made of a highly conductive metal, usually aluminum, coated with a photoconductive, often selenium. A photoconductor is simply a material that will hold a charge in darkness, but that loses the charge when exposed to light. The surface of the cylinder is electrically charged, and then a bright lamp is passed over the image. The area of the original image that is blank white will reflect light back onto the cylinder, discharging those areas. Where the image is dark, however, the photoconductor remains charged. The result is a sort of electric map of the original image. Some more advanced copiers produce a digital copy of the image and use a laser to charge the cylinder.
The next step is to apply toner to the electrically charged cylinder. Toner is likely powdered ink, and it is attracted to the charged regions, where it sticks. The static electricity that holds the toner to the cylinder is the same force that makes hair stick to the surface of a balloon if the two are rubbed together long enough to produce a charge. All that remains is to transfer the image to a sheet of paper, where the toner is melted by heat and pressure rollers, and to "erase" the image on the cylinder by exposing its entire surface to light.
The mechanics of a photocopier are modest, but still somewhat complex. The image transfer does not occur at the site where the original image is placed. Instead the drum must be deep within the machine, in complete darkness, until a series of mirrors project the image onto it. A belt moves the photoconductor to the toner and brings it together with the paper. The cylinder is erased by a second set of lights and is made ready to be electrically charged again for a new image.