How graphics tablets work.

I find it quite amazing graphics tablets can be both cordless and battery-less. If you're wondering how they work, here's how:

1. The tablet sends power to the pen using induction. This is similar to those contactless chargers or bus cards.
2. The pen uses the power to send a signal with data on pressure and button states.
3. The tablet locates this signal to a precision of 1/1024 inch.

It's like a game of Marco Polo, with the tablet going "Marco" and the pen going "Polo", then the tablet finds out where the pen is. The interesting thing is, the pen only has to be near the tablet, it doesn't have to be touching it - that means you can put things on the tablet and trace them. Or put a screen protector on the tablet...
Why don't they make wireless mice like this?