page_title: Balance wheel
section_title: History
hierarchical_section_title: Balance wheel / History
caption_attribution_description: English: Drawing of the bottom section of Giovanni de Dondi's astronomical clock, the Astrarium, finished 1364, Padua, Italy. This early weight-driven clock kept time with a balance wheel (crown shape, at top) driven by a verge escapement (wheel under it). This is one of the earliest existing drawings of those mechanisms. The balance wheel had a beat of 2 seconds. The actual clock was a lot more complicated than this drawing, which only shows the lower section containing the weights, escapement and main gear train. The upper section, mounted on top of this, had 7 dials, displaying the astrological motions of the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn. See Giovanni De Dondi's Astrarium. Alterations to image: converted to 8 color PNG.
context_page_description: 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 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. 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.
context_section_description: The balance wheel appeared with the first mechanical clocks, in 14th century Europe, but it seems unknown exactly when or where it was first used.  It is an improved version of the foliot, an early inertial timekeeper consisting of a straight bar pivoted in the center with weights on the ends, which oscillates back and forth.  The foliot weights could be slid in or out on the bar, to adjust the rate of the clock.  The first clocks in northern Europe used foliots, while those in southern Europe used balance wheels.  As clocks were made smaller, first as bracket clocks and lantern clocks and then as the first large watches after 1500, balance wheels began to be used in place of foliots.  Since more of its weight is located on the rim away from the axis, a balance wheel could have a larger moment of inertia than a foliot of the same size, and keep better time.  The wheel shape also had less air resistance, and its geometry partly compensated for thermal expansion error due to temperature changes.
caption: Perhaps the earliest existing drawing of a balance wheel, in Giovanni de Dondi's astronomical clock, built 1364, Padua, Italy. The balance wheel (crown shape, top) had a beat of 2 seconds.  Tracing of an illustration from his 1364 clock treatise, Il Tractatus Astrarii.
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