A hairspring is a slender coiled spring that regulates the motion of the balance wheel in a timepiece.
The hairspring (balance spring) is an extremely fine coiled spring that works with the balance wheel as the watch's regulating organ. It stores and releases energy with each oscillation, controlling the balance wheel's beat rate. Made from Nivarox or silicon alloys, it must maintain consistent elasticity across temperature variations and resist magnetic fields to ensure accurate timekeeping.
A longer effective hairspring results in slower oscillations (watch runs slow); shorter runs fast. Watchmakers adjust rate by moving the regulator pins to change the active length. This is the primary method of rate adjustment. The Breguet overcoil terminal curve improves isochronism by allowing the spring to breathe more concentrically, reducing position errors.
Silicon hairsprings (used by Patek Philippe, Rolex's Syloxi, Omega, Rolex) are manufactured using microelectronics processes with extreme precision. Silicon is non-magnetic, requires no lubrication, resists temperature variations better than traditional alloys, and can be machined to complex 3D geometries impossible with metal. The trade-off is brittleness — silicon cracks under severe shock unlike flexible metal springs.