To illustrate a weird rotational device.
Celts as pictured.
When the celt is rotated in its normal direction, it continues to rotate smoothly as it slows down due to friction. If it is rotated in the opposite direction, it begins to rock violently end-for-end, slows down, then reverses its direction of rotation, ultimately rotating smoothly in its normal direction. When it is at rest, tapping one end to make it rock results in rotation in its normal direction. ◙Celts are constructed with an S-shaped keel line which leads to this weird behavior. Pictured above are small clockwise and counterclockwise celts along with a large clockwise celt. Small commer
(PIRA unavailable.) See Demonstration Reference File for additional information, including papers claiming◙to explain this effect. Celts are sometimes referred to by the term "rattleback."◙