Balance wheel

A balance wheel, or balance, is the timekeeping device used in mechanical watches and small clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a spiral torsion spring, known as the balance spring or hairspring. It is driven by the escapement, which transforms the rotating motion of the watch gear train into impulses delivered to the balance wheel. Each swing of the wheel (called a "tick" or "beat") allows the gear train to advance a set amount, moving the hands forward. The balance wheel and hairspring together form a harmonic oscillator, which due to resonance oscillates preferentially at a certain rate, its resonant frequency or "beat", and resists oscillating at other rates. The combination of the mass of the balance wheel and the elasticity of the spring keep the time between each oscillation or "tick" very constant, accounting for its nearly universal use as the timekeeper in mechanical watches to the present.

Primitive balance wheels appeared in the first mechanical clocks in the 14th century, but its accuracy is due to the addition of the balance spring by Robert Hooke and Christiaan Huygens around 1567. Until the 1980s virtually every portable timekeeping device used some form of balance wheel. Since the 1980s quartz timekeeping technology has taken over most of these applications, and the main remaining use for balance wheels is in mechanical watches.