A balance wheel is the timekeeping device used in mechanical (vintage) watches and some clocks, similar to the pendulum in a pendulum clock. The balance wheel is a mechanism that, while having the same isochronous quality as the pendulum, is not so susceptible to vibrations and is therefore portable. The balance wheel comprises a fine “hairspring” fitted into a metal wheel, with one end of the spring fixed to the wheel and the other fixed to the body of the timepiece. Instead of the back-and-forth motion of a pendulum, the balance wheel rotates around the axis of the wheel in an alternating clockwise and anti-clockwise motion. The angle of the hairspring’s rotation is called the oscillation angle. Like the pendulum, the hairspring is isochronous, so the cycle remains unchanged even when the oscillation angle shifts. The invention of the balance wheel made it possible to produce timepieces that could remain accurate even on board of a (rocking) ship or when carried on a wrist or pocket. 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 nearly universal use as the timekeeper in mechanical watches to the present.
Lao Tsu, Chapter 11 in the Tao Te Ching (TAO TE CHING:) reads:
"Thirty spokes unite at the single hub (of the wheel);
It is the empty space which makes the wheel useful.
Mold clay to form a bowl;
It is the empty space which makes the bowl useful.
Cut out windows and doors;
It is the empty space which makes the room useful."
Another translation reads:
"We join spokes together in a wheel, but it is the center hole that makes the wagon move.
We shape clay into a pot, but it is the emptiness inside that holds whatever we want.
We hammer wood for a house, but it is the (empty) inner space that makes it livable."
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