The balance wheel is the part that keeps time in a mechanical watch or small clock. It is a weighted wheel that swings back and forth, pulled back each time by a spiral spring. It does the job a pendulum does in a pendulum clock. Its swinging sets how fast the gears and hands move.
How does a balance wheel work?
The wheel carries weight and swings first one way, then the other. A fine spiral spring, the hairspring, pulls it back towards its resting position each time. Together, the wheel and hairspring work as a harmonic oscillator. Each swing is one beat, and two beats make one full cycle. How long that cycle takes depends on the wheel’s moment of inertia. Put simply, that is how strongly the wheel’s weight resists being turned. Common rates are 18,000, 21,600, 28,800 and 36,000 beats per hour.
Where it comes from
Simple balance wheels appear in the earliest mechanical clocks of the 1300s. They grew out of the foliot, an earlier timekeeping part that also relied on inertia. The first balances had no spring, and watches could lose or gain hours each day. In 1658 Robert Hooke became the first to fit a metal spring to the balance. In 1674 Jean de Hautefeuille and Christiaan Huygens gave the balance spring the spiral shape it still has today. With the spring in place, watches became far more accurate.
Where is it used today?
Since the 1980s, quartz technology has taken over most of the jobs the balance wheel once did. Mechanical watches are now its main use. In a modern watch the balance is usually made of Glucydur. This alloy expands little when heated, so the wheel changes size very little as it warms.
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