Balance Systems

The balance and its hairspring together form the watch's time base, and their combined design has been the single most studied problem in precision watchmaking since the eighteenth century, because the accuracy of everything downstream — the escapement's counting, the gear train's translation into hands — depends entirely on how consistently this oscillator keeps its period. A plain, uncompensated steel balance changes its moment of inertia slightly with temperature (metal expands and contracts) and, combined with a hairspring whose elasticity also varies with temperature, produces a rate that drifts across a temperature range unless compensated.

The bimetallic compensation balance, refined through the nineteenth century, addresses this by cutting the balance rim into two arms, each made of two fused metals (typically steel inside, brass outside) with different expansion rates, so the arms curl inward or outward with temperature change in a way calculated to offset the hairspring's own thermal drift. These balances are visually distinctive, with cut rims and small adjustable timing screws around the circumference, and are one of the identification features covered in the historical-construction lesson.

From the early twentieth century, monometallic balances made from special low-expansion alloys (marketed under names such as Glucydur or Nivarox for the balance and hairspring respectively) achieved better and more stable compensation without a cut, bimetallic rim, simplifying manufacture and improving consistency; this is the type found in the great majority of watches made from the mid-twentieth century onward, recognisable by an uncut, solid-looking rim.

Poising a balance means adjusting its weight distribution (traditionally by removing tiny amounts of metal from timing screws or the rim) so its centre of mass coincides exactly with its pivot axis; an out-of-poise balance runs at different rates in different orientations (dial up versus pendant down, for instance) because gravity then exerts an uneven torque as the balance rotates, which is one of the main causes of positional rate variation in an otherwise well-adjusted watch.

The Breguet overcoil — a hairspring whose outer coil is raised and curved back over the spring before terminating at the stud — improves isochronism by making the spring breathe more concentrically as its coils expand and contract, correcting a geometric flaw inherent in a flat spiral spring. Overcoils are more difficult to manufacture and adjust than flat springs, which is why they are associated with higher-grade watches and remain, even now, a mark of careful hand or high-precision finishing rather than a purely cosmetic feature.