A
balance wheel, or
balance, is the timekeeping device used in
mechanical watches and some
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, 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 near universal use as the timekeeper in mechanical watches to the present. From its invention in the 14th century until tuning fork and
quartz movements became available in the 1960s, virtually every portable timekeeping device used some form of balance wheel.