· By Buffy
Frequency
18,000 vs 28,800 vs 36,000 vph: what beat rate buys you, what it costs, and why 4 Hz became the world's default.
A watch’s frequency is how fast its balance oscillates. The classic spec is vph — vibrations (semi-oscillations) per hour. Halve it and divide by 3600 to get full beats per second:
| vph | beats/sec | Hz (full oscillations) | era / use |
|---|---|---|---|
| 18,000 | 2.5 | 2.5 Hz | vintage, pocket, slow-beat luxury |
| 21,600 | 3 | 3 Hz | vintage auto, some modern |
| 28,800 | 4 | 4 Hz | the modern default |
| 36,000 | 5 | 5 Hz | high-beat (El Primero, GS hi-beat) |
| 43,200–72,000 | 6–10 | 6–10 Hz | experimental / observatory (Breguet 7727 at 10 Hz) |
What a faster beat buys
- Disturbance dilution. Each impulse is a shove; at 8 shoves per second instead of 5, each shove is smaller relative to the momentum. External disturbances (wrist motion) are averaged out faster.
- Resolution. Seconds hands sweep more smoothly; chronographs (read) can subdivide beats further (1/5, 1/10 s displays are beat-rate tricks).
- Rate stability in practice. High-beat movements tend to hold position rates closer together — less spread, better real-world averages.
What it costs
Energy and wear scale with beat rate: more impulses per hour means more amplitude drain per hour, more pallet wear, more lubrication demand, shorter service intervals — and less power reserve per milligram of mainspring. 18,000 vph movements can run 60+ hours easily on modest springs; 36,000 vph is a thermodynamic lifestyle choice.
Why 4 Hz won
28,800 vph splits the difference with unusual elegance: fast enough for stable rate and smooth sweep, slow enough for 40–70 hour reserves and long service intervals. The industry converged on it through the 1970s–90s — the same convergence that made calibres like the Valjoux 7750 ubiquitous.
Quartz and the joke
Quartz oscillators run at 32,768 Hz — the reason a $10 quartz watch beats a $10,000 chronometer. But watch people count swings of a balance, and there is something to it: a quartz crystal is a sliver of rock on a circuit, while a 28,800 vph balance is a wheel on a spring that has been arguing with gravity since Huygens. Frequency is a physics choice; the choice of oscillator is a philosophy.