Amplitude

Amplitude is the angle, measured in degrees, through which the balance wheel rotates away from its rest position on each swing. It is not a rate figure and says nothing directly about whether a watch keeps good time. It describes how vigorously the oscillator is being driven, which is why a watchmaker reads it before almost anything else. A balance running at 300 degrees is swinging most of a full circle away from rest in each direction, reversing eight times a second on a 28,800 vph caliber, and it is that stored swing energy that lets the escapement shrug off friction and small shocks instead of being pushed around by them.

How a timing machine reads it

A timegrapher never sees the balance. It listens. A microphone picks up the noises each escapement action makes, the unlocking, the impulse and the drop of the escape wheel, and times the interval between the first and the last. That interval stretches as the balance swings further, so the machine converts it into an angle.

To do the conversion it needs one number that belongs to the caliber rather than to the watch: the lift angle, the arc through which the balance turns while impulse is actually being delivered. Published values cluster between about 40 and 55 degrees, and 52 degrees is the default a machine falls back on if told nothing. ETA publishes 49 degrees for the Valjoux 7750. Enter the wrong figure and the amplitude line is out by tens of degrees while the watch is unchanged. Phone apps measure rate usefully and amplitude poorly, because a phone microphone rarely separates those noises cleanly. Our timegrapher reading explainer reads a full trace line by line.

What good looks like

A healthy modern movement typically shows somewhere around 270 to 310 degrees dial-up at full wind, measured 10 to 15 minutes after winding so the mainspring has settled.

Vertical positions read lower, often by 20 to 50 degrees, because the balance pivots carry weight sideways instead of resting on their ends. A drop much beyond 50 degrees points at the pivots or their jewels rather than at the mainspring. Amplitude also falls steadily as the mainspring unwinds, so a reading taken 30 hours into a 40 hour power reserve is not comparable with one taken at full wind. A common bench criterion is that a movement should still hold roughly 200 degrees 24 hours after full wind.

Why it matters

Amplitude falls before the rate misbehaves, which makes it an early warning. Degraded oil, a dirty pivot, a tired mainspring, a bent balance staff or a movement that has been shocked will all show up as a droop in amplitude first. A watch reading around 200 degrees or less at full wind is asking for attention, whatever the numbers on the rate line say.

There is a second reason. No real escapement is perfectly isochronous, so rate is partly a function of amplitude. A watch whose swing sags through the day drifts through the day, which is why a movement can look excellent on a bench and time poorly on a wrist that never winds it fully.

What to do about a low reading

Rule out the cheap explanations first. An automatic worn a few hours a day may never reach full wind, so hand wind it where the caliber allows before measuring. Check the lift angle. Then consider magnetism: a magnetized hairspring whose coils cling together pulls amplitude down as well as sending the rate sharply fast, and demagnetizing takes a couple of minutes. Our guide to magnetism covers the symptoms.

If the reading stays low, it is a bench problem, not an adjustment problem. Regulation on a timing machine, indicatively 60 to 150 US dollars before sales tax, moves the rate and does nothing for amplitude. Restoring amplitude means cleaning, fresh oil and often a new mainspring: indicatively 250 to 500 dollars for a three-hand automatic at an independent. See how often a watch should be serviced.

Common misconceptions

Amplitude is not a quality ranking, and chasing the highest figure is not the goal. Some calibers are designed to run lower, and a low-beat movement behaves differently from a high-beat one. It is also not frequency: vibrations per hour is how often the balance swings, amplitude is how far.

Nor is it beat error, the symmetry of the two half swings, or rate, the only line that tells you whether the watch keeps time. A timegrapher shows all three at once and they answer different questions. Amplitude has no meaning for a quartz watch, which has no balance to swing and is diagnosed by current draw instead. See timing your own watch for how to gather readings worth comparing.