Some 50 years ago, before the copier was invented, if you wanted to duplicate a document, you had to recopy everything by hand! Today it is amazing to think that, in mere seconds, you can produce an exact replica of what’s on a sheet of paper simply pressing the start button of a copier! This article focus on the basics of the technology behind the copying process.
At its heart, a copier works because of one basic physical principle: opposite charges attract.
Inside a copier there is a special drum. You can charge it with a form of static electricity. The drum, charged with static electricity, is used to attract a very fine black powder known as toner. The drum can be selectively charged, so that only parts of it attract toner.
To produce photocopies of an original document, the photocopy machine first makes a temporary image, a sort of negative of the original– in static electricity — on the surface of the drum. How is this accomplished? By using a photoconductor — a material that will hold a charge in darkness, but that loses the charge when exposed to light. The surface of the cylinder is electrically charged, and then a bright lamp is passed over the image. The area of the original image that is blank white will reflect light back onto the cylinder, discharging those areas. Where the image is dark, however, the photoconductor remains charged. The result is a sort of electric map of the original image. Because the process involve light, that is why the machine is called a photocopier!
The drum that is selectively charged will then selectively attracts the toner creating a map of the original image. Then the sheet of paper gets charged with static electricity and it pulls the toner off the drum. Since toner is heat sensitive, the loose toner particles are melted by heat and pressure rollers and fused to the paper.
In order for the process to work well, the drum must be deep within the machine, in complete darkness, until a series of mirrors project the image onto it. A belt moves the photoconductor to the toner and brings it together with the paper. The drum is erased by a second set of lights and is made ready to be electrically charged again for a new image.