Carlson copied with static charge and dry powder, no wet chemicals

In the 1930s making a copy of a document usually meant photography: taking a negative, then printing it, with wet chemicals, darkrooms and time. Chester Carlson, a patent attorney, saw how much people needed a fast, cheap way to copy a page and set out to build one. Instead of trying to improve the photographic process, he looked for a different principle entirely.

His insight was to combine photography with static electricity. A material like amorphous selenium is a photoconductor: it holds an electric charge in the dark and conducts the charge away where light strikes it. Carlson would charge the surface, project the image of a page onto it, and the white areas would lose their charge while the dark image areas kept theirs, leaving an invisible latent image of the page made of charge.

To make that image visible he dusted the plate with toner, a fine powder of the opposite charge, which stuck only to the charged areas of the latent image. He then pressed paper against the plate and applied heat to fuse the powder in place. The whole process was dry — no liquid chemicals, no darkroom — and produced a copy in a few steps and seconds.

He was awarded U.S. patent 2,297,691 for the process, which he called electrophotography and which was later renamed xerography, from the Greek for dry writing, on October 6, 1942. More than a dozen companies turned the idea down before Haloid Photographic Company, later Xerox, took it on and turned it into the office copier.