I have a
magic stone. Oblong and deep gray, smoothed by years of massage by a local stream, the stone nestles well in a worried palm. That by itself makes it
special, but not magical.
One day, while idly spinning it, it awoke. It spun a few circles, shuddered to a stop, then reversed itself and spun in the opposite direction.
I spun it again. Again, it rattled to a stop, and spun the other way. Spin. Wobble and rattle. Counterspin. I found myself a ratttleback.
Rattlebacks have a long history. Stones of similar shape have been found buried in Egyptian tombs (hey, a good toy helps pass away eternity), and people have played with them long before anyone understood Newtonian physics.
The rattleback is also known as a "wobblestone" (for obvious reasons), and as a "celt." As per the OED Celt comes from the Latin word celtes, meaning stone chisel, and is used to describe certain chopping tools used by prehistoric peoples. I do not know when the rattleback came to be called a celt.1
You can find rattlebacks sold as scientific toys, sometimes at outrageous prices. Some rattleback merchants wrongly tout the toy as a model demonstrating the coriolis force. Even some physicists have a time wrapping their minds around the toy's mechanics. Sir Hermann Bondi wrote: "Many people, even trained scientists, find it hard to understand that the behaviour of the toy doesn't violate the principle of conservation of angular momentum." 2
Rattlebacks deserve recognition as one of the all-time classic toys, in the same class as Jacob's ladders, spinning tops, and marbles.
1 I imagine bored
archaeologists spinning celts as they sit in tents during bad weather at their archaelogical sites. "Why, look, Professor James, this bloody implement is possessed!" If anyone has an idea of the real etymology, I would be much obliged.
2 Quote taken from http://www.tam.uiuc.edu/toys/celt/, a nice little site from the University of Illinois at Urbana-Champaign College of Engineering, that has a video clip showing a rattleback in action. For a more involved mathematical explanantion, I recommend http://www.autolev.com/WebSite/SampleProblemRattleback/Rattleback.html.