The hairspring, or balance spring, was invented by Dutch physicist Christiaan Huygens in 1675. Huygens realised that pairing a balance wheel with a spiral spring produced an oscillator with a predictable, repeatable frequency, analogous to a pendulum but immune to orientation. The hairspring's restoring force, combined with the balance wheel's inertia, defines how often the watch ticks. Change the hairspring's stiffness by even a fraction of a percent, and the watch gains or loses seconds per day.
The Breguet overcoil, developed by Abraham-Louis Breguet around 1795, curled the outer end of the hairspring upward and inward in a second plane. This solved an isochronism problem: a flat hairspring breathes unevenly at the extremes of its travel, introducing rate errors at different amplitudes. The overcoil forces the coil to breathe concentrically, keeping rate constant across amplitudes. The modern equivalent, the Phillips curve (Edouard Phillips, 1861), mathematically derives the ideal overcoil geometry.
The magnetism problem dogged hairsprings for 250 years. Traditional steel hairsprings magnetise easily and thereafter lose accuracy until demagnetised. Nivarox (developed by Reinhard Straumann in 1933) is a copper-iron-nickel alloy that is mostly antimagnetic and forms the hairspring of the vast majority of Swiss movements today. Rolex's Parachrom blue hairspring (2000) is a niobium-zirconium alloy completely antimagnetic. Tudor's Nivachron (2018) is a titanium-based alloy developed jointly with the Swatch Group that is both antimagnetic and shock-resistant.
The silicon revolution began in 2001, when Ulysse Nardin put a silicon escapement wheel in the Freak, followed by Patek Philippe's Spiromax silicon hairspring in 2005. Silicon is completely antimagnetic, extremely light (allowing thinner, faster oscillators), and can be photo-etched to precise shapes including built-in terminal curves that eliminate the need for Breguet-style overcoils. Rolex's Syloxi silicon hairspring (2014) is used in their ladies' movements. Silicon is now the high-end standard, though Nivarox and Parachrom continue to dominate by volume.
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