A Xerox machine seems a miraculous thing: it makes a clear, permanent copy of a document on ordinary paper in about 5 seconds and the copy is sometimes easier to read than the original.
Like many modern inventions, the Xerox copier should not be possible under the classical laws of physics laid down by Isaac Newton.
The machine depends for its miracle on the manipulation of particles smaller than an atom and an understanding of light that did not exist until physicists Max Planck and Albert Einstein explained it at the turn of the century. They proved that light behaves like a stream of particles, called photons.
Since then, technology has discovered the semiconductor, a substance that ordinarily does not conduct electricity but can do so under certain conditions.
Electricity is a flow of electrons; in the atoms of a semiconductor, electrons are usually too tightly bound to their nuclei to flow when a current is applied to the material. But when struck by photons vibrating at a certain frequency, the substance becomes a good conductor.
This happens because each photon “kicks” an electron away from its nucleus, and the electrons are then free to flow. Compounds containing silicon, selenium, arsenic, germanium, or sulfide have this property. In a Xerox machine, a cylinder coated with semiconductor material receives light, forms a pattern of conducting and nonconducting atoms, and transfers the pattern to a piece of paper to make an image.
When, you place a document facedown on the glass top of a Xerox copier and press the “print” button, an aluminum cylinder beneath, coated with layers of the semiconductors selenium and arsenic selenide, begins to turn. An electrode sprays the cylinder with a temporary layer of positively charged particles that are pulled from molecules of air around the machine. Charged particles are called ions; the Xerox image is formed from a pattern of positive ions and neutral particles on the surface of the cylinder.
A light beams up through the glass and across the surface of your document, and the image reflects off the page and back down at the cylinder. The white parts of the original bounce most of the light photons striking it onto the rolling cylinder, whereas the black areas of the page absorb light instead and reflect no photons onto the corresponding areas of the cylinder.
It is at this stage that the semiconductor coating works its magic. Wherever a vibrating photon reflects off the page and strikes a semiconductor atom on the cylinder, that atom conducts electricity by “kicking” an electron in the atom away from its nucleus. Electrons have a negative charge; bear in mind that opposite electrical charges attract each other, and like charges repel.
Once away from its nucleus, each negatively charged electron is attracted to the layer of sprayed on positive ions outside the semiconductor layer of the cylinder; the electron combines with one ion and neutralizes it. An electric current applied to the aluminum inside the cylinder replaces the electrons “kicked” from the semiconductor atoms.
Where the original page is white, the photons thus create an electrically neutral zone on the cylinder; the black areas of the page leave their corresponding unexposed regions positively charged, where no electrons flowed from the semiconductor to neutralize the positive ions.
The black pigment or toner in a Xerox copy, it’s not ink, is made of tiny black spheres less than a millionth of an inch in diameter, called BBs by Xerox scientists.
These BBs have a strong negative charge; while they are stored waiting to be used, they adhere to larger “carrier” BBs, which are positively charged. The purpose of the carrier BBs is simply to transfer toner from the reservoir to the image. After the image is flashed onto the surface of the cylinder, the cylinder turns until it pushes against the supply of toner. The positive charge of the unexposed areas of the cylinder is much stronger than the charge of the large BBs, so the small negative BBs are more strongly attracted and hop off onto the positive regions of the cylinder.
Now the image formed in tiny BBs of toner, which adhere wherever the original document was black, is ready to print. Another quarter turn of the cylinder brings it to a sheet of paper that has been given an even more powerful positive charge than the cylinder’s. The BBs hop off as they touch the paper. Then the paper is heated and pressed to melt the toner into its surface, and a finished copy finally emerges from the machine, still hot and full of static electricity, but hard to distinguish from the original within a few minutes.
Meanwhile, the cylinder moves past a cleaning brush, which takes any remaining toner off its surface, and then is blasted with light to erase the old image and make the surface ready for the next copy.
In a Xerox machine, photons (light particles) strike the cylinder (whose surface is represented here in cross section) and “kick” electrons away from the layer of semiconducting selenium atoms (center layer).
Those electrons, which are negatively charged, are attracted to the layer of positive ions above. Each electron combines with one ion, creating a neutral, uncharged, particle (black disks). The aluminum layer below feeds new electrons to the selenium atoms to stabilize them wherever electrons have been kicked away.
The positive ions that remain on the cylinder after exposure (white disks) will attract negatively charged black toner and transfer it to the paper in the printing process. The neutral zones (black disks) will attract no toner and leave white areas on the copy.