The balance spring — also called a hairspring or, in technical English parlance, hairspring — is the component that governs the oscillation frequency of the balance wheel and, consequently, the precision of the entire movement. Without it, the balance wheel would oscillate without rhythm: it is the spring that recalls it to center after each excursion, defining the tick of the second. This is watchmaking work, not assembly: whoever has never adjusted a hairspring by hand does not know what it truly means to touch a movement.
The material it is made from changes everything. For decades, tempered steel was the standard, but steel is ferromagnetic and sensitive to temperature variations: two enemies of daily precision. The industry's answers have come from different directions. Rolex developed Parachrom, a proprietary alloy of niobium and zirconium that appears blue after oxidation, with resistance to magnetic fields significantly superior to traditional steel. For more recent movements — caliber 2232, 1908 Perpetual, Land-Dweller — Rolex switched to Syloxi, a paramagnetic silicon hairspring that adds thermal stability and practically eliminates magnetic drift. ETA and the Swatch Group adopted Nivachron, a non-ferromagnetic alloy of niobium and titanium, mounted on widely distributed calibers like the Powermatic 80 and Rado's R766. Pure silicon, used by Breguet in flat-hairspring calibers, by Patek Philippe in the Silinvar, and by H. Moser & Cie in the HMC 805, takes the concept even further: extremely lightweight, self-lubricating, immune to magnetism.
Shape matters as much as material. The standard flat hairspring, if not constructed with care, introduces isochronism errors: the coils draw closer together during large amplitudes and alter the oscillation period. Breguet solved this with the overcoil curve that bears his name — the Breguet overcoil — which brings the last section of the hairspring to a higher plane, making expansion more symmetrical. The terminal curve, in its Phillips or Gustav Gerstenberger variants, further refines this principle by correcting the curvature of the final section. The cylindrical hairspring, used in complex axial tourbillons like the Jaeger-LeCoultre Duométre Hélïotourbillon, is yet another solution: by coiling on a cylinder rather than a plane, it offers more uniform behavior regardless of orientation.
A double hairspring — two coils superimposed and offset — appears in some high-precision tourbillons, including Moser's HMC 805: the two springs compensate for each other, reducing friction and isochronous drift. It is an expensive solution and difficult to regulate, but when it works you hear it in the tick: cleaner, more stable.
From the bench, after fifty years, I can say only one thing: the balance spring is the component that separates a watch that works from one that runs well.