Balance Spring
The fine spring attached to the balance wheel that controls its oscillation and helps determine the movement’s rate.
Also known as: hairspring, hair spring, balance hairspring, spiral
Definition
The balance spring is the extremely fine spiral spring attached to the balance wheel. As the balance rotates, the spring alternately coils and uncoils, providing the restoring force that returns the balance and creates a repeating oscillator. Its elasticity, effective length and the balance's inertia determine the oscillation period. Traditional springs are metallic alloys, while some modern movements use silicon. Geometry, temperature behaviour, magnetism and how concentrically the spring breathes can all influence rate stability.
Why it matters
Small changes in the balance spring can produce measurable rate changes, so its material, shape, adjustment and resistance to magnetism are important to precision and long-term stability.