Apparently the MagnaDoodle was a very popular present this year! If you have kids, then you are familiar with this toy. For the kid-less, a quick description: A MagnaDoodle is a drawing toy. It has a white screen and a "pen" that allows you to draw on the screen. As you draw, a very nice, dark black like forms within the white screen (this is weird and amazing to see in action -- check it out next time you pass a Toys 'R Us). There is a slider at the bottom of a MagnaDoodle, and by sliding it back and forth once or twice you erase the screen instantly and completely (also amazing).
Little kids love MagnaDoodle, I think, because the drawn line is very bold. However, no pressure on the pen is required to create the line -- a light touch draws the same line as pressing hard (unlike crayons). Parents love MagnaDoodle because the pen contains neither ink nor paint to spill. The tip of the pen is a blunt piece of metal, so if the artist tries to write on a wall nothing happens.
There is a very nice Java MagnaDoodle simulator on this page if you'ld like to try one out!
So how does a MagnaDoodle work? There are four basic parts:
When you apply a small magnet -- the blunt metal tip of the pen -- to the front screen, it draws the black magnetic particles through the liquid to the surface and they become visible. They don't sink back into the liquid because of the thickness of the liquid and because the particles and liquid are matched to have the same density. To erase the screen, the slider moves a long magnet across the back of the screen. This magnet drags the magnetic paticles toward the back screen and the white liquid covers them!
- There are two sheets of plastic, laid one on top of the other. The top one is translucent and forms the surface that the pen touches when drawing.
- Between these two sheets is a honeycomb lattice that acts as a separator. The lattice is visible when you look at the screen. The lattice keeps the two sheets spaced out at a uniform thickness and also divides the interior between the sheets into cells.
- In the cells between the sheet there is an opaque white liquid (think of thick milk)
- Mixed with the liquid there's very fine black magnetic particles of iron oxide.
Here are the two U.S. Patents that cover MaganDoodle: