Balance Wheel

Definition

The oscillating wheel that governs a mechanical watch's rate, doing the job a pendulum does in a clock. It swings back and forth on its hairspring, and each swing lets the escapement release one step of the train. The rate is quoted in vibrations per hour: 28,800 vph, or 4 Hz, is the modern standard, used by the Rolex 3235 and the Breitling B01; Omega runs its co-axial calibers at 25,200 vph; Patek Philippe's 240 and most Audemars Piguet automatics beat at 21,600 vph, which draws less energy; and the Lemania-based CH 27-70 Q still runs at 18,000. A faster beat resists disturbance better; a slower one lets a small barrel run longer.