Q: How do copy machines make copies?

A: Dry copiers like the Xerox use a process called xerography which was invented in the 1950s. It uses the photoelectric effect, static electricity and heat to fuse a plastic toner into the shapes of letters and pictures.

The photoelectric effect results when light strikes a metal surface. A photon of light can knock electrons away from a metal if the photon has more energy than is required to hold the electron to the atoms of the metal.

Certain metals hold on to their electrons weakly enough so that photons with energies found in light in the visible range can knock them away. A metal of this type is used for the drum of the dry copier.

In the copier a bar of light moves down the page to be copied at the same rate that the metal image drum revolves. Light is reflected and focused from the page onto the drum leaving a line by line negative image on the drum where the electrons have been knocked away from the lighter portions of the document.

The missing electrons leave a region of positive charge on the drum which would quickly dissipate. But before the charge can dissipate, the rotating drum contacts the paper on which the image will form. Electrons flow from the paper to the areas of positive charge on the drum, leaving a region of positive charges on the paper which matches the dark areas on the original page. A positive latent image now exists on the paper.

The charge on the paper dissipates more slowly than the charge on the drum. But before it dissipates, the paper is dusted with the toner which adheres to it where the positive charges are now arranged in the same pattern as the dark areas on the original.

The toner is a mixture of finely ground carbon particles, like soot, and fine grains of a plastic resin which melts at low temperatures.

Next the toner is melted quickly as the paper containing the image is pressed between a pair of hot rollers just before exiting the machine.

The entire process is mechanized like an assembly line, and is controlled by an electronic microprocessor. The electromechanical system is quite complex, and major advances have been made since the first Xerox machine was introduced in 1960.

Richard Brill is assistant professor of science at Honolulu Community College. Send questions to Honolulu Community College, 874 Dillingham Blvd., Honolulu, HI 96817, fax to 239-5152 , email to rickb@hcc.hawaii.edu Visit Science Inquiry on the internet at http://www.hcc.hawaii.edu/~rickb/SciDoc.html

Xerography 1996 Richard C. Brill