David Kramer, age 13, of Farfield, Conn., for his question:     

HOW IS A XEROGRAPHIC COPY MADE?

A great deal of today's printing is done on huge machines that press an inked plate onto a clean piece of paper. The ink is transferred to the paper, and the result is a book, picture, newspaper or whatever is being printed. Xerography is one method of electrostatic printing that reproduces material without ink or pressure. Instead, this method uses light, a special powder and a plate coated with a substance called selenium. Selenium is a very useful chemical element. One of its special properties is the ability to conduct electricity when light shines on it.

When you push the button on a copying machine, a rotating drum coated with selenium gets an electrostatic charge. A light goes on, and the material that is being copied is projected through a lens onto the selenium drum. This produces a reverse image that is then dusted with a toner powder. The toner powder only sticks to the drum where the image was projected. Paper is fed into place and briefly heated to melt the powder, producing a copy. Of course, while the entire process takes only seconds to complete, it is a bit more complicated than it sounds and depends on negative and positive charges on the selenium drum, the toner powder and the paper.