The Rattleback

The rattleback, or celt, is one of those science toys that seems so simple, yet behaves in a highly counterintuitive way. It has fascinated people of all ages-- and challenged even accomplished scientists.

Smooth stones with the odd behavior displayed by the celt have been discovered by peoples around the globe. Basically, when spun one way about its vertical axis, the celt will spin for a long time. When spun the other way, however, an ever amplifying wobble sets in ultimately bringing the spin to a halt and then -- almost miraculously - reversing it! In his key paper of 1986 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." That's the attraction - this simple device, this toy, displays behavior that deviates from what we have learned to expect from similar devices spun over a lifetime of empirical observation.

Make your own in the Practical of the week.
Watch a video of one (QuickTime 2.8 MB).
Learn the physics of the rattle back on this link.