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Gestetner upright office photocopier with paper drawers beneath.

Photocopiers

by Chris Woodford. Last updated: July 13, 2014.

Big companies sometimes make big mistakes. When American inventor Chester Carlson (1906–1968) approached some of the world's largest corporations with his idea for a photocopying machine, during the 1940s, they simply didn't want to know. They couldn't imagine who would want to make lots of copies of documents. It took Carlson years to turn the idea into one of the most important office inventions of the 20th century—and those companies kicked themselves when they realized just how big an opportunity they'd missed. Photocopiers look complex, but they work using two pretty simple pieces of science. Let's take a closer look inside!

Photo: A typical photocopier in a public library. This one is made by Gestetner and, unlike many office copiers, doesn't have a sheet feeder built into the top (because it's primarily used for copying books). The finished paper copies curl through the mechanism and appear in the empty space you can see underneath. The drawers at the bottom hold spare paper.

Static electricity: a neat kind of glue!

A girl's hair blows out with static when she touches a Van der Graaf generator

Have you ever tried that party trick where you rub a balloon on your pullover 20 or 30 times? If you rub enough, you can make the balloon stick to your clothes all by itself. What you see isn't magic: it's static electricity. When you rub the balloon, you give it an electrical charge. At the same time, you give your jumper an opposite electrical charge. Unlike charges attract, so the balloon sticks to you.

Photo: Static electricity can make your hair stand on end, as in this classic school experiment with a Van der Graaf generator. The same science is put to practical use inside a photocopier. Photo courtesy of Sandia National Laboratories/US Department of Energy.

How does this happen? As you rub the balloon, electrons (the tiny negatively charged particles inside atoms that carry electricity) move from your pullover onto the balloon. In other words, the balloon gains more electrons than it should have and picks up an overall negative electrical charge. Since the electrons have left your pullover, it has fewer electrons than it should have and an overall positive electrical charge. Now things with an electrical charge are a bit like magnets. Two objects with an opposite electrical charge tend to move toward one another, or attract, just like two magnets with opposite poles. (Our article on static electricity explains all this in much more detail.)

What's light got to do with electricity?

Static electricity is one of the two scientific tricks that makes a photocopier work. Now let's explore the other: photoconductivity.

If you believe what you read in science books, you probably think light and electricity are totally different things. Light comes from the Sun and powers things like flashlights; electricity flows round wires and makes things like vacuum cleaners and refrigerators work. So light has nothing to do with electricity, right? Wrong! Light is actually a kind of electricity. A ray of light is an ultra-fast wave of electricity and magnetism wiggling back and forth and zapping through space. That helps us to explain how solar power (making electricity from sunlight) works. When sunlight shines onto a solar panel, the solar cells inside it soak up the electrical energy in the light and convert it back into an electrical current (flow of electrons) that can be used to power something.

There's something similar to a solar cell in a photocopier and it's called a photoconductor. Instead of producing an electric current when light shines onto it, it captures the pattern of the light as a pattern of static electricity. What use is this? Suppose you shine a flashlight at your hand to cast a shadow image of a rabbit's ears on the wall. But instead of shining the shadow on the wall, you shine it on a photoconductor. Some parts of the photoconductor will be brightly lit (where the light passes around your hand) and some parts will be dark (where your hand casts a shadow). The photoconductor will gain an electrical charge where it is light and no charge where it is dark. In other words, it will have a kind of "electrical copy" of your hand. This is the key to how a photocopier works.

Writing with light

After a great deal of research and tinkering in his laboratory, Chester Carlson figured out how he could use these two bits of science—static electricity and photoconductivity—to help him make copies of documents.

Suppose you want to copy a page from a book. If you shine an extremely bright light on the book, you can make a shadow of the black and white characters on the page, just like casting a shadow of your hand. If you shine the light onto the page at an angle, it doesn't reflect straight back: it bounces off at an angle. So, by shining the light at an angle, you can throw a shadow of the page onto another object. Let's suppose you put a photoconductor nearby and throw the image of the page onto that. You won't create a shadow on the photoconductor—you'll make a pattern of electrical charges: an electrical version of a shadow. Now if we sprinkle ink powder over the photoconductor, toner particles will stick to the charged areas of this "electrical shadow" like tiny little balloons sticking to your pullover. All we have to do then is press a piece of paper onto the photoconductor to lift the ink away. Hey presto, the paper has a copy of the original page! This whole process, which Carlson named xerography (combining two Greek words to mean "dry writing"), is automated inside a photocopier and can happen over and over again very quickly.

In case that's not clear, I'll go through it all again, exactly as it happens inside the copier, in the box below.

How a photocopier works

How a modern copier works

How a photocopier mechanism works. Artwork illustration showing the component parts of a xerox-type copier work

  1. You place the document you want to copy upside down on the glass
  2. An extremely bright light scans across the document. Much more light reflects off the white areas (where there is no ink) than off the black, inked areas.
  3. An "electrical shadow" of the page forms on the photoconductor. The photoconductor in a photocopier is a rotating conveyor belt coated with a chemical called selenium.
  4. As the belt rotates, it carries the electrical shadow around with it.
  5. An ink drum touching the belt coats it with tiny particles of powdered ink (toner).
  6. The toner has been given an electrical charge, so it sticks to the electrical shadow and makes an inked image of the original page on the belt.
  7. A sheet of paper from a hopper on the other side of the copier feeds up toward the first belt on another conveyor belt. As it moves along, the paper is given a strong electrical charge.
  8. When the paper moves near the upper belt, its strong charge attracts the charged toner particles away from the belt. The image is rapidly transferred from the belt onto the paper.
  9. The inked paper passes through two hot rollers (the fuser unit). The heat and pressure from the rollers fuse the toner particles permanently onto the paper.
  10. The final copy emerges from the side of the copier. Thanks to the fuser unit, the paper is still warm. It may still have enough static electric charge to stick to your pullover. Try it (but make sure the ink is dry first).

How Chester Carlson's original copier worked

Internal mechanism of Chester Carlson's original photocopier, from his US patent 2,357,809 granted in September 1944.

What happens in a modern copier isn't so very different from the process that Chester Carlson originally designed. We can see that by taking a look at one of the design drawings from his original photocopier patent. I've colored and simplified the numbering to show roughly what's happening:

  1. You insert the original document (to be copied) into a slot (green) at the top.
  2. The document is carried into the machine by a belt and roller mechanism (dark blue).
  3. A bright lamp (yellow) shines through the document and transfers an electrical shadow of it onto the photoconductor (orange).
  4. The photoconductor is mounted on the outer surface of a drum (red), which carries the image past the toner hopper and brush.
  5. Toner is now attracted from the hopper onto the charged parts of the drum.
  6. You insert a blank sheet of paper into a slot on the opposite part of the machine. Carried inside by rollers (purple), it picks up the inked image from the drum.
  7. A fuser unit heats, presses, and seals the image into the paper and the finished copy emerges.

As you can see from all the dozens of small numbers on the original drawing, this is a very simplified account of what's really happening—and all the parts and pieces that are involved. You can read a much more detailed explanation by taking a look at Chester Carlson's patent, listed in the references below. Although technical, it's very readable and fairly easy to understand.

Artwork: Chester Carlson's original photocopier from his patent granted on September 12, 1944. Courtesy of US Patent and Trademark Office.

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Text copyright © Chris Woodford 2007, 2012. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2007) Photocopiers. Retrieved from http://www.explainthatstuff.com/photocopier.html. [Accessed (Insert date here)]

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