· By Buffy
Power Reserve
A gauge for the mainspring: differentials, indicator scales, and why a power-reserve display is harder to build than it looks.
A power-reserve indicator (réserve de marche) shows how much energy remains in the mainspring — typically 40 to 70+ hours on a small arc. It is the watch’s fuel gauge, and mechanically it is a subtraction problem.
The differential
The barrel winds in one direction (via the crown or an automatic rotor) and unwinds in another (running the gear train). A needle that merely followed the barrel would swing wildly during winding.
The solution is a differential: a planetary gear that outputs the difference between how far the barrel has unwound and how far it has been wound. The hand shows remaining travel — going to zero as the reserve dies. Assembly requires hand-calibrating the zero point against the actual stop: the least envious job on the bench (as noted earlier).
Design shapes
- Linear scales (F.P. Journe’s Réserve de Marche, A. Lange & Söhne’s outsize date family) — an arc or straight scale sweeping 0 to max.
- Sub-dial gauges — vintage instrument style.
- Retrograde hands that snap back to zero when rewound.
Automatic movements complicate the picture: the rotor is always topping up, so the display oscillates in daily wear rather than sweeping monotonically down. Owners learn to read the trend.
Why it matters beyond romance
Amplitude falls as the reserve empties, and with it (rate-wise) goes isochronism. A power reserve display is therefore the only complication that tells you when your watch’s accuracy will change. Long reserves (70h+, 120h, 8 days) are the modern luxury arms race — often won by twin or series-coupled barrels (constant-force thinking) rather than magic.
More in What Counts as a Complication.