Photocopiers
by Chris Woodford. Last updated: May 27, 2011.
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 office photocopier with a sheet feeder built into the top. It works in a similar way to the paper feed on the top of an inkjet printer, with little metal teeth and rubber rollers that drag one sheet of the original document through the copying mechanism at a time.
Static electricity: a neat kind of glue!
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.
