The hairspring is the small spring whose elasticity keeps the balance oscillating. Its inner end is fixed to the balance staff and its outer end to the balance bridge. Two innovations stand out in its history: the curve devised by Abraham-Louis Breguet in 1795, and silicon, which arrived in watches in the mid-2000s (Breguet).
Huygens's flat hairspring
Breguet recalls that the flat hairspring was invented in 1675 by the Dutch physicist and astronomer Christiaan Huygens. This first spring, in copper or iron, had only a few coils and its isochronism left much to be desired. Yet it gave the balance what it needed to become a true regulator. Our article on isochronism explains why that regularity matters so much.
1795: the coil that lifts
In 1795 Abraham-Louis Breguet had the idea of raising the outer coil and bringing it back towards the centre along a calculated curve (Breguet). With this "Breguet curve", the hairspring develops concentrically: the watch gains accuracy and the balance staff wears more slowly.
The Fondation Haute Horlogerie explains the principle: on a flat hairspring, all the coils lie in the same plane; a spring whose outer coil is raised is called an overcoil. This shape keeps the centre of gravity of the balance and spring at the centre during oscillations. Among the watchmakers who defined ideal curves, the Foundation names Breguet, then Édouard Phillips.
The house of Breguet adds that between about 1880 and 1910, many factories engraved "spiral Breguet" in large letters on the cuvette of their watches. The name had become a selling point.
2006: silicon
In 2006 Breguet presented its first watches with a silicon hairspring and escapement. The house describes the material as insensitive to magnetic fields, lighter than conventional hairsprings, less deformed by gravity than a metal spring and highly resistant to corrosion (Breguet).
The same year Patek Philippe introduced the Spiromax® hairspring in Silinvar®, a silicon-based material, on the Annual Calendar Ref. 5350. The manufacture credits it with resistance to temperature variations, lightness, no need for lubrication and insensitivity to magnetism.
A Breguet overcoil in silicon
Silicon does not bend like metal. Giving such a part an end curve that rises above the spring forced Breguet to rethink the whole manufacturing process. The result was unveiled in 2010 in the Tradition 7047, a fusee-and-chain tourbillon (Breguet). Two centuries after 1795, the same idea of concentric development had found a new material.
Frequently asked questions
How do you recognise a Breguet overcoil?
By its last coil, raised and curving back towards the centre above the others.
What about alloys such as Nivarox?
Our article on Guillaume's Invar and Elinvar tells the story of these alloys, and the one on the cylindrical hairspring the quest for accuracy at sea.
Further reading
The School's Breguet hairspring module studies the curve in detail. To understand the pair the hairspring forms with the balance, read our article on the balance wheel, then join the Décalibre School.