· By Buffy
Amplitude
The single number that tells you if a watch is healthy: how far the balance swings, what's normal, what's dangerous, and how to read it off a timegrapher.
Amplitude is how many degrees the balance wheel rotates in each direction of its swing — from its rest point to the extreme of the arc. It is the vital sign of a mechanical watch: the first number a watchmaker reads and the one that tells the truth fastest.
What’s healthy
For a modern wristwatch movement in dial-up position, full wind:
| Amplitude (dial up, full wind) | Reading |
|---|---|
| 270–310° | healthy, fresh service |
| 220–270° | acceptable, aging oil or partial wind |
| below 220° | trouble: friction, magnetism, or dirt |
| above ~315° | overbanking risk — see below |
Vertical positions (crown down etc.) run 30–60° lower than dial up; that spread is normal. Vintage movements run lower overall; judge against their era, not a modern spec sheet.
Why it moves around
Amplitude is the balance between energy in (the escapement’s impulse) and energy out (friction, air drag). Anything that changes either side shows up here first:
- Falling mainspring torque — amplitude sags across the power reserve.
- Old oil — viscosity rises, amplitude falls, rate drifts with it.
- Magnetism — coils touching, amplitude collapses and the rate races.
- Position — pivots bear differently; dial up wins because gravity assists concentric rotation.
Overbanking: too much of a good thing
When amplitude exceeds roughly 315°, the balance’s impulse jewel can arrive at the fork early and throw the escapement into a false unlock — the wheel spins a few teeth and relocks. Result: a watch that suddenly gains many minutes (“rebanking”). Causes: over-winding a hand-wound watch that’s already adjusted for full wind, excessive impulse, or a weak mainspring bridle letting torque spike. It’s the one failure mode where more energy is the enemy.
How to measure it
A timegrapher listens to the tick and infers amplitude from the phase of the beat against a reference — you must tell it the movement’s lift angle (45–58° typical) or the number it prints is fiction. Compare positions, compare full wind vs. +24h, and watch for the sag signature of tired oil.
Amplitude is information; rate is opinion.