When you put a piece of printed paper into a copying machine, a lens focuses the letters onto a drum or plate coated with a metal that is very sensitive to light.
This metal has an electric charge, but the electric charge disappears from the areas of the drum that the printing isn’t focused on, in other words, it disappears from all the “white” spaces on the page.
Next, the machine spreads a powder over the surface of the drum. This powder is electrically charged too, but it has a charge opposite to the charge of the metal on the drum. Since opposite electric charges attract each other, the powder sticks to all parts of the drum that are still charged, that is, to all the parts covered by the printing.
Finally, a piece of paper is pressed against the drum. Then an electric charge under the paper attracts the powder left on the drum. Since the powder had stuck only to the parts of the drum covered by the printing, the paper now contains a duplicate of the original page. Heat fuses the powder to the paper, and a finished copy rolls out of the machine.
Some copying machines are so fast that they can turn out two copies per second!
