Amplitude scales with the lift angle you enter, so an angle that is too small inflates the reading. The failure is not the wrong number; it is the service that does not happen.
The amplitude trap: a wrong angle hides a tired movement
issue ISSUE 01written 2026-08-26responsible Escapement Papers (a named bench, not a person)
tape card: the direction of the error
| Angle error | Amplitude reported | What it hides |
|---|---|---|
| Too small | Too high | A tired train |
| Too large | Too low | A healthy movement, wrongly condemned |
| Correct | Correct | Nothing |
The arithmetic, worked
Amplitude is computed from the fraction of the beat the balance spends crossing the lift angle. If you tell the machine the angle is 44 degrees when it is really 48, the machine thinks the balance crossed a shorter arc in the time it measured, and concludes it is swinging further. The reading comes out high by roughly the ratio of the two angles.
| True angle | Angle entered | True amplitude | Reported amplitude |
|---|---|---|---|
| 48 degrees | 48 degrees | 250 degrees | 250 degrees |
| 48 degrees | 44 degrees | 250 degrees | about 273 degrees |
| 48 degrees | 52 degrees | 250 degrees | about 231 degrees |
| 48 degrees | unknown | 250 degrees | unusable |
Why the high direction is the dangerous one
A reading that is too low sends a healthy watch for a service that it did not need. That is a cost. A reading that is too high keeps a tired movement in use, which is a wear problem that gets worse and eventually costs more than the service would have. The bench's concern is the second case, and it is the one an unset default produces.
A quick sanity check: a well-made watch at full wind rarely exceeds about 310 degrees of amplitude. A reading above that is more likely to be a wrong angle than an exceptional movement.
A sentence to argue with
The bench holds that an amplitude of 270 degrees with an undisclosed lift angle is not a measurement, it is a rumour. Some watchmakers will argue that a default angle is close enough for a service decision; the bench's answer is that the default is wrong by more than the width of the decision band on a large share of modern movements.
References
- Ref 1Stud (Hairspring Stud), Horopedia – Watchmaking Encyclopediasupports the stud whose position affects the rate the amplitude is read with
- Ref 2Bearings (Jewels), Horopedia – Watchmaking Encyclopediasupports the jewelled bearings whose friction shows up as amplitude loss
- Ref 3Mainspring, Fondation de la Haute Horlogerie encyclopediasupports the mainspring as the source of the energy amplitude measures
One link, one document, titled as it titles itself. No home pages, no search results.
Set this page down Set this page down when you have set the angle and re-read the amplitude.