Crookes Radiometer

I have long been fascinated by radiometers.  Usually they resemble a light bulb (the one pictured is more of a hanging Christmas ornament) with a four spoked vane inside.  Each vane has diamond shape at the end; one side black, the other white.  Shine a light on the device, and the vane starts spinning.  Bright sunlight can send the vane spinning furiously!  On a cloudy day, even the proximity of your hand can affect its movement.

In my life radiometers were a fixture of grade school science  classrooms, so imagine my surprise when I learned that the workings of it are still a bit of a mystery!  Instead of puzzling through the physics myself, I've linked the Wikipedia article that discusses the radiometer in action.  I guess that I'm a bit of a Luddite for just thinking of radiometers as interesting kinetic decor.

Solve the mystery by getting your own radiometer HERE
Wikipedia entry on the radiometer