Check out this wheel of purple fire!

It's not everyday that you combine weird physics with defunct medical equipment and get a success. (Although I would love to see one of those old tobacco-smoke-up-the-butt bellows filled with nitroglycerin.) The thing that looks like a pizza-cutter is a Wartenburg Pinwheel, an old piece of neurological equipment that measures nerve response. It allows a doctor to slowly and systematically run it along a person's body to check for numbness or sensitivity. Today, because it is so difficult to properly clean, it has mostly been retired from medical work.

Check out this wheel of purple fire!

The little plumes of purple fire shooting out of it are corona discharge. Essentially, enough of an electrical potential differential will ionize the air nearby. The electrons in the air will come loose from their atoms and swim around in a plasma. This makes it very easy for electricity to move through the air. The energy of it all excites the air molecules which give off a glow. In this case, the glow shoots from each of the spikes of the wheel.

But Wheel of Purple Fire sounds better.

Image: Ultrapurple