Power reserve indicator
A display showing how much mainspring tension remains
A differential gear reads the gap between the barrel wall and arbor rotation, driving the indicator hand from full to empty.
What it is
The power reserve indicator displays the current state of wind of the mainspring, telling the wearer how many hours remain before the watch stops running. It is typically shown as a sector hand sweeping from "full" to "empty" on a dedicated sub-dial, or as a linear bar display. On automatic watches, where the rotor winds the mainspring continuously during wear, the display is most useful for indicating when the watch has been off the wrist long enough to need hand-winding.
History
Power reserve indicators appeared in pocket watches from the 18th century. In wristwatches they became a practical feature in thin automatic movements where the owner cannot feel the mainspring winding under the rotor. IWC, A. Lange & Söhne, and Jaeger-LeCoultre popularised them in the modern era. The Lange 1's asymmetric dial layout incorporates a power reserve display as one of its four off-centre indications. Movements with shorter reserves; some 38–40-hour dress calibers; benefit most from a visible indicator, since a watch stored over a weekend may arrive at Monday morning barely wound.
How it works
A differential gear senses the relative rotation between the mainspring barrel wall and the barrel arbor, which changes as the spring winds and unwinds. The differential converts this difference into a rotation that drives the power reserve display hand or bar. As the mainspring unwinds from full to empty, the indicator sweeps from maximum to minimum over the rated power reserve period. The display is non-linear: the first and last portions of the mainspring wind produce less consistent torque, so the indicator may spend more time at certain positions.
Parts required
Differential gear (reading barrel tension from the difference between barrel wall and arbor rotation), power reserve transmission wheel, sector or linear display wheel, indicator hand
In the catalog
Related
- Mainspring: The coiled steel strip that stores the watch's energy
- Barrel: The cylindrical container for the mainspring
