7 Chester F. Carlson 7

Chester F. Carlson

(b. Feb. 8, 1906, Seattle, Wash., U.S.—d. Sept. 19, 1968, New York, N.Y.)

A

merican physicist Chester F. Carlson was the inventor
of xerography (from the Greek words meaning
“dry writing”), an electrostatic photocopying process
that found applications ranging from office copying to
reproducing out-of-print books.
By the age of 14 Carlson was supporting his invalid
parents, yet he managed to earn a college degree from
the California Institute of Technology, Pasadena, in 1930.
After a short time spent with the Bell Telephone Company,
he obtained a position with the patent department of P.R.
Mallory Company, a New York electronics firm.
Plagued by the difficulty of getting copies of patent
drawings and specifications, Carlson began in 1934 to look
for a quick, convenient way to copy line drawings and text.
Since numerous large corporations were already working
on photographic or chemical copying processes, he turned
to electrostatics for a solution to the problem. The basis
of the process is photoconductivity, an increase in the
ability of certain substances to allow an electric current to
flow through them when struck by light. The chemical
element selenium, for example, is a poor electrical con-
ductor, but when light is absorbed by some of its electrons
and a voltage is applied, these electrons are able to pass
more freely from one atom to another. When the light is
removed, their mobility falls. Xerography typically uses an
aluminum drum coated with a layer of selenium. Light
passed through the document to be copied, or reflected
from its surface, reaches the selenium surface, onto which
negatively charged particles of ink (i.e., the toner) are
sprayed, forming an image of the document on the drum.
A sheet of copy paper is passed close to the drum, and a