INERTIA

Inertia is the property of a body to resist any change in its state of movement. In watchmaking, the key concept is the moment of inertia of the balance, which measures the distribution of the mass around the axis of rotation. With the elasticity of the hairspring (balance spring), the moment of inertia of the balance determines the frequency of the oscillations, and thus the timekeeping of the watch: the higher it is, the longer the period is long (runs slower), and conversely. The Regulation / Adjustment of the rate can thus be achieved by acting on the Inertia rather than on the Length active length of the Hairspring (balance spring): this is referred to as a variable-inertia Balance, fitted with small weights or Screws located at the periphery of the Felloe (rim base); screwing them inward decreases the Inertia (advance), moving them outward increases it (delay). This peripheral adjustment, more stable than a Regulator, equips many high-quality modern Movements. A large Balance with a high Moment of inertia, combined with a controlled Frequency, promotes chronometric stability, provided that the driving torque is sufficient to sustain it.

Historically, the variation of the Moment of inertia was also used for thermal compensation: marine chronometers employed a Bimetallic Balance whose Rim, cut at two points, carried weights that moved closer to or further from the Axis according to the differential Expansion of the strips. This principle, perfected by Arnold and Earnshaw at the end of the 18th century, lost its importance after the discovery of elinvar by Charles-Édouard Guillaume (Nobel Prize in Physics 1920). In today’s industry, some calibers even dispense with any Regulator assembly: the Frequency of the Balance-Hairspring (balance spring) pair is adjusted directly at the factory using a laser process.