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.
Static electricity and photo conductivity are the two scientific tricks that makes a photocopier work. Now let’s explore them: After a great deal of research and tinkering in his laboratory, Chester Carlson figured out how he could use these two bits of science 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 photo conductor nearby and throw the image of the page onto that. You won’t create a shadow on the photo conductor—you’ll make a pattern of electrical charges: an electrical version of a shadow. Now if we sprinkle ink powder over the photo conductor, 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 photo conductor 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.
How a modern copier works:
- You place the document you want to copy upside down on the glass
- 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.
- An “electrical shadow” of the page forms on the photo-conductor. The photo-conductor in a photocopier is a rotating conveyor belt coated with a chemical called selenium.
- As the belt rotates, it carries the electrical shadow around with it.
- An ink drum touching the belt coats it with tiny particles of powdered ink (toner).
- 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.
- 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.
- 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.
- 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.
- 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).