A mechanical-watch balance wheel oscillates back and forth driven by impulses from the escapement; on each oscillation the wheel swings from its centre rest position out to its maximum angular extent on one side, then through centre to maximum extent on the other side. The maximum angular distance from centre is the amplitude; it is measured in degrees of arc (the wheel's arc, not the watch case).
A healthy modern automatic movement at full wind shows 270-310° dial-up. This is the operational target for a freshly-serviced watch and the spec range for chronometer certification. As the mainspring unwinds through the power reserve, amplitude drops naturally: a 70-hour reserve typically shows ~290° at full wind and ~230° at the end of reserve. The drop is gradual and predictable; rate is regulated to stay within tolerance across the curve.
"Amplitude tells you what the watch will do tomorrow. Rate tells you what it does today."- Watchmaker on timegrapher diagnostics
The diagnostic value of amplitude is foundational. Below 240° at full wind indicates a service is needed: lubricant has dried, components are dirty, or the mainspring has weakened. Below 200° the watch is at risk of stopping in vertical positions; below 180° the escapement may not deliver enough impulse to maintain oscillation and the watch will stop intermittently. Position drop (the difference between dial-up amplitude and vertical-position amplitude) is normally 30-60°; larger drops suggest worn jewels or unbalanced poising.
Knocking is the opposite problem: amplitude exceeding 340° means the balance is swinging so far that the impulse pin overruns the safety pin on the back-swing, striking the safety pin from the wrong side. The audible signature is a sharp tick on every other beat; the rate becomes unstable. Causes: a too-strong new mainspring, incorrect main-train friction, or a balance that has lost its proper inertia. Fix is at servicing.
For buyers and owners, amplitude is the most useful single number for evaluating a vintage or used mechanical watch. A timegrapher measurement showing 270-300° dial-up at full wind = the watch is healthy; 240-260° = service due soon; below 240° = service overdue. Most independent watchmakers will run a free timegrapher reading on request; printouts are standard practice for service handover. Free-sprung balances tend to show slightly different amplitude curves than index-regulated balances; both are evaluated on the same scale.