GRAVITY

Definition

Gravity is the fundamental physical force by which two masses attract each other. On Earth, it manifests as the weight of objects and their tendency to fall or be drawn toward the center of the planet. In watchmaking, gravity is not only an external constraint: it has also, historically, been a source of energy and a disruptive factor that had to be mastered.

A driving force: weight-driven clocks

Before the widespread adoption of the mainspring, gravity was the main means of driving a timepiece movement. Weight-driven clocks harness this force directly: one or more driving weights, suspended from cables or chains wound around drums, slowly descend under their own mass and transmit this downward motion to the train (wheel train). The weight thus takes the place of the spring: the heavier it is, the more torque it delivers to the train (wheel train). This principle of a gravity-driven oscillator is also found in the pendulum: it was by observing the isochronism of the oscillations of a hanging chandelier that Galileo, at the beginning of the 17th century, laid the foundations for its use as a regulating organ, later exploited by Huygens in the first pendulum clocks. Finally, gravity plays a role in modern mechanical horology as a source of energy: in an automatic watch, the oscillating weight (rotor) converts wrist movements, under the effect of gravity, into rotation that winds the mainspring.

A disruptive factor for the regulating organ

In a wristwatch, gravity changes role: it becomes a source of disruption for the regulating organ (balance-hairspring). The latter, supposed to oscillate at a frequency constant regardless of the orientation of the watch, is in reality influenced by its own weight and that of its associated components. This influence is particularly noticeable when the balance is in a vertical position (dial up, stem to the left, stem down, etc.): gravity then exerts an asymmetric action on the balance, the hairspring (balance spring) and the escapement mechanism, which can slightly alter the rate. In horizontal position (dial facing up or down), this influence is largely neutralized. This sensitivity to position, known as positional error, is one of the components of isochronism of a movement: a perfectly compensated regulator would maintain the same frequency regardless of its orientation.

Fighting its influence: tourbillon and carousel

To mitigate rate deviations linked to gravity, watchmakers have developed devices that rotate the entire escapement and balance-hairspring (balance spring) assembly on itself, generally once per minute or every few minutes. By distributing the effects of gravity across all vertical positions successively, these mechanisms statistically compensate for deviations instead of adding them up in a single orientation. The tourbillon, invented to address this problem, is the most well-known example. The carousel serves the same purpose through a different architecture, with a cage driven independently of the main train (wheel train), which makes it easier to manufacture by eliminating the need for extreme lightness of the cage.

Good to know

These devices do not eliminate the influence of gravity: they average it out over time. A watch equipped with a tourbillon or a carousel can therefore remain sensitive to gravity if it stays motionless in a single position, for example resting on a table rather than being worn on the wrist.