tourbillon

thumb|280px|Tourbillon movement (:File:Stuhrling Tourbillon Movement.ogv|high resolution)

In horology, a tourbillion () or tourbillon (; "whirlwind") is an addition to the mechanics of a watch escapement to increase accuracy. Conceived by the British watchmaker and inventor John Arnold, it was developed by his friend the Swiss-French watchmaker Abraham-Louis Breguet and patented by Breguet on 26 June 1801. In a tourbillon, the escapement and balance wheel are mounted in a rotating cage, with the goal of eliminating errors of poise in the balance giving a uniform weight.