Photocopying

A process somewhat similar to that used in xerography is electrostatic copying. The major difference between these two processes is that in electrostatic copying, the endless photoconducting surface is omitted from the machine and the copy paper is specially treated to pick up the toner.

The paper used in electrostatic copying is treated with a material consisting of zinc oxide combined with a thermoplastic resin. When that paper is fed into the copy machine, it is first passed through a corona charging bar, similar to the one used in xerography. Within the charging bar, the zinc oxide coating picks up a negative electrical charge.

In the next section of the copy machine, the original document is exposed to light, which reflects off the white portions of the document (as in a xerographic machine). Dark portions of the document, such as the letter "a" in the document, do not reflect light. Light reflected off the original document is then reflected by a series of mirrors to the treated copy paper which has been passed into this section of the machine. Light striking the copy paper removes negative charges placed by the charged bar, leaving charged sections that correspond to the absence of light, that is, the dark places on the original document. In this respect, the copying process is exactly like that which occurs in xerography.

Next, the exposed copy paper is passed through a toner bath, where positively-charged toner attaches itself to negatively-charged areas on the copy paper. When the paper is passed through a pair of rollers, the toner is pressed into the copy paper, forming a permanent positive image that corresponds to the image on the original document.

The electrostatic copy process became less popular when xerographic processes were improved. The main drawback of the electrostatic process was the special paper that was needed, a kind of paper that felt different from ordinary paper and was more expensive to produce and to mail.