A balance spring, or hairspring, is a spring attached to the balance wheel in mechanical timepieces. It causes the balance wheel to oscillate with a resonant frequency when the watch is running, which controls the speed at which the wheels of the watch turn, thus the rate of movement of the hands.

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In other words, the hairspring provides restoring force to the balance wheel, enabling isochronal oscillations. It is a flat spiral spring that breathes at each vibration of the balance wheel. The spring itself was originally made from steel or gold, and today is made from temperature resistant alloys or silicon.

The hairspring and balance wheel together are the regulating organ of the mechanical watch. This harmonic oscillator is very resistant to outside disturbances, which makes it especially suited to keeping track of time. The balance wheel and hairspring is very similar to the pendulum in a clock. The major difference is that the balance wheel and hairspring are portable, while the pendulum is not.