Tourbillon

In horology, a tourbillon (/tʊərˈbɪljən/; French: [tuʁbijɔ̃] "whirlwind") is an addition to the mechanics of a watch escapement. Developed around 1795 and patented by the French-Swiss watchmaker Abraham-Louis Breguet on June 26, 1801, a tourbillon aims to counter the effects of gravity by mounting the escapement and balance wheel in a rotating cage, to negate the effect of gravity when the timepiece (thus the escapement) is stuck in a certain position. By continuously rotating the entire balance wheel/escapement assembly at a slow rate (typically about one revolution per minute), positional errors are averaged out.

Originally an attempt to improve accuracy, tourbillons are still included in some expensive modern watches as a novelty and demonstration of watchmaking virtuosity. The mechanism is usually exposed on the watch's face to show it off.

Gravity has a direct effect on the most delicate parts of the escapement, namely the pallet fork, balance wheel and hairspring. Most notably is the hairspring, which functions as the for the escapement and is thus the part most sensitive to any exterior effects, such as magnetism, shocks, temperature, as well as inner effects such as pinning positions (inner collet), terminal curve, and heavy points on the balance wheel.

Many inventions have been developed to counteract these problems. Temperature and magnetism problems have been eliminated with new materials. Shocks have much less effect today than at Breguet's time thanks to stronger and more resilient materials. The oscillator still gets disturbed at the moment of the shock, but the hairspring does not get as easily deformed from shocks as before, and so re-stabilizes itself quickly after such an event.

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Wikipedia