Before the advent of the photocopying machine, simple and cheap copying methods were both messy and crude. Older readers may remember the "Hectograph" from public school days. The master copy was written on a piece of paper with a special ink. The master was pressed face down on a tray of gelatine which dissolved and absorbed the ink. Moist blank sheets of paper were pressed onto the inked gelatine and a few inferior copies could be transferred this way.
The "Ditto" process is a variant on the Hectograph. By typing or writing, a special ink impression is made on the back of the master sheet. This master is mounted on a drum in a machine like a small printing press. On each rotation of the drum, methyl alcohol dissolves a bit of the ink and it is transferred to a blank sheet of paper. Twenty-five or so copies may be made.
The "Gestetner" process produces a master by typing, or writing with a stylus, on a special wax-impregnated paper. The wax is removed where the typewriter strikes it, making the paper porous to ink. Again, a small, rotary printing press is used. It is messy but can produce many copies.
In 1934, Chester Carlson, a physicist turned patent attorney, decided there must be a better way and embarked on a programme of research which led to the development of Xerography and the modern photocopier. The first commercial unit was the Zerox-914 which appeared in 1960.
There are a few substances such as selenium, arsenic, and tellurium which are "photoconductors." These substances are electrical insulators in the dark and are conductors in the light. A thin layer of this material is coated on a metal backing either in the form of a drum or a flat plate. At the start of the copy cycle, this drum is given a uniform static electric charge either by rubbing it or spraying on the charge.
Next, the page to be copied is illuminated with a strong light and an image of the page is formed by a lens on the charged photoconductor. White areas thus light up the photoconductor and it is rendered conducting so that its charge is leaked away to the metal backing. This leaves a latent image of the black areas of the page formed in the electric charge on the plate. This latent image must now be developed.
Originally, Carlson wafted a dust of black particles (called "toner") over the plate. Now small glass or plastic beads loaded with the black toner are either rolled or gently brushed over the plate. The toner sticks only where there is remaining charge; i.e., on the black areas of the original copy.
Next, a blank sheet of paper is rolled over the plate and the toner is transferred to the paper forming a perfect image of the original. This image, however, is very fragile and must be "fixed" or made permanent. This fixing is done in different ways, but one of the most common is by means of heat. The toner is actually a black coloured thermoplastic which, on being heated, melts and impregnates the paper. In other copiers, a chemical spray dissolves the toner so that it can impregnate the paper. Finally, the photoconductor plate is cleaned and ready to be recharged for the next cycle.
Machines in which the toner is wafted on, tend to deposit it at the edge of the charged regions so that large black areas are reproduced as black edges with white centres. The brushing method produces a more uniform toner deposition and thus reproduces large black areas more uniformly.
http://www.physics.uoguelph.ca/summer/scor/articles/scor54.htm
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