Balance spring

The balance spring is the regulating heart of any mechanical movement: without it, the balance wheel would oscillate without rhythm and the watch would measure nothing. I call it a hairspring when I want to be precise, because that is exactly what it is: an extremely thin ribbon wound in a flat spiral, anchored internally to the collet and externally to the hairspring stud. The oscillation frequency depends on its rigidity, its mass, and its active length. Modifying any one of these parameters means shifting the regulation.

The historical problem with the hairspring is its sensitivity: to mechanical wear, to temperature variations, to magnetic fields. Metallurgy answered first. Glucydur and then Nivarox reduced thermal drift. Nivachron—a niobium and titanium alloy developed by ETA—added resistance to magnetic fields without sacrificing the elastic characteristics of steel. You find it today in ETA Powermatic 80 movements and in Rado calibers like the R766. The practical result is a movement that doesn't lose seconds if you bring it near a speaker or a magnetic clasp.

Silicon changed the game in the 2000s. Rolex introduced the Parachrom—their own alloy, not pure silicon—and then the Syloxi, in paramagnetic silicon, mounted on the 2232 caliber and movements in the 1908 family. Breguet works with flat silicon. Jaquet Droz and H. Moser use silicon hairsprings with double superimposed laminae: the so-called double hairspring, which improves isochronism and reduces friction distortion between adjacent coils. Silicon requires no lubrication, is paramagnetic, and can be laser-cut to tolerances impossible for traditional metals. The downside I know from direct experience: it is brittle. A sharp impact on a silicon hairspring can break it; one in Nivarox bends and often recovers.

The geometry of the terminal curve is a separate chapter. The terminal curve—called the Phillips terminal curve or Breguet curve depending on the profile—modifies the outer closure of the hairspring to make it more isochronous: oscillation remains constant regardless of amplitude. Without this correction, the watch runs differently depending on the state of wind. Moritz Grossmann uses a curve calibrated according to Gustav Gerstenberger's designs; Chopard declares the Phillips terminal curve on the LUC caliber. These are details that make real difference on the chronometer.

The mainspring—the one inside the barrel—is a different component, often confused with the hairspring. The barrel is the container that stores energy; the hairspring is the oscillator that governs it. Between the two is the train, the anchor, the escapement gear. When Dietrich Gruen patented the Safety Pinion in 1874, he protected the train precisely from barrel breakage: if the spring failed suddenly, the safety pinion would disengage before the energy destroyed the downstream mechanism.