Photocopiers
Last updated: September 23, 2008.
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. This one's made by Gestetner. Other popular makes include
Canon, Ricoh, Minolta, and Kyocera.
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.
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 things with an opposite electrical charge tend to
move toward one another, or attract, just like two magnets with
opposite poles. That's why the balloon sticks to your pullover. The
Science Made Simple website has an excellent page on
static electricity
that explains all this in much more detail. Static electricity is one
of the two scientific tricks that makes a photocopier work. Now let's
explore the other: photoconductivity.
What's light got to do with electricity?
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.
