A Xerox copier uses static electricity to copy documents. The photoreceptor is charged positively, and light is shone on the document to neutralize the areas to be copied. Toner is then applied to the positively charged areas, transferred to paper, and fused with heat. The process is repeated for multiple copies.
A Xerox® copier, collectively known as a copier, operates essentially on the principles of static electricity. Electric charges can be positive or negative, which is somewhat similar to the poles of magnets. The negative end of one magnet will instantly stick to the positive end of another magnet, but two negatives or positives repel each other. The same thing happens with negative and positive electric charges; a negatively charged comb will attract positively charged pepper flakes or salt grains.
What a Xerox® copier does is exploit this natural electrical attraction or repulsion. What we see as a document to be copied is actually seen as nothing but black or white space on a Xerox® copier. When the document is placed on the glass tray for copying, several processes are performed simultaneously. This is where the science of photocopying begins.
Beneath the clear glass tray is a flat belt or round drum called a photoreceptor. The surface of this photoreceptor consists of special materials that can be negatively and positively charged in different areas at the same time. An electrical element known as a corona wire moves through the photoreceptor and imparts a positive electrical charge to the entire surface, much like rubbing a latex balloon or running a comb through your hair. Once the entire surface of the photoreceptor is positively charged, an intensely bright light moves across the surface of the original document to be copied.
This light emits photons that can easily penetrate the white sections of the document, but not the dark areas we see as writing. When photons hit the photoreceptor surface, those areas are electrically neutralized. The photoreceptor areas under the dark areas of the document, however, still remain positively charged. Essentially, a Xerox® copier “paints” the drum or photoreceptor ribbon with static electricity, creating a patch of images of positively charged letters surrounded by negatively charged areas that will remain white.
Once the document image has been statically “painted” onto the drum or ribbon, the Xerox® Copy System coats the plate with a negatively charged black powder called toner. Wherever toner particles find positively charged areas on the photoreceptor, they will stick together like pepper flakes on a statically charged comb. The toner will not stick to the negatively charged areas of the ribbon or drum, so those areas will remain white.
After the toner particles have settled in the positively charged areas, a second sheet of paper is placed over the photoreceptor. A Corona wire passes over this paper to give it a positive charge. The toner particles are transferred to the loaded paper and now resemble the original document image. However, the toner is still in the form of a loose powder, so the new copy is pressed through a series of heated rollers known as a fuser. The heat from the fuser fuses the toner to the paper permanently. This is why some Xerox® copies, especially those made at the end of a long print run, can feel very hot to the touch.
After each copying cycle is completed, a Corona wire essentially purges the photoreceptor drum or ribbon by carrying a new positive charge. Since many photoreceptor drums are smaller in diameter than the length of the copy paper, it is not uncommon for the photocopying process to be duplicated multiple times when scanning the original document. It may be necessary to positively recharge the drum to discard older information and receive new images several lines at a time. All of these actions are carefully timed to produce a photocopy in the time it takes for the lamp to scan the original once.
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