Power reserve indicator
Power reserve indicators (known as “Réserve de Marche” in French) were developed alongside long power reserve movements which can run for more than 2 days. Typically shown on the dial with a pointer, power reserve indicators are also sometimes located on the movement itself, visible through a display caseback or aperture. They are often geared directly to the winding system and usually use a gearing mechanism based on estimated power for a given number of winding turns. Others are attached to the barrel and use mainspring torque as an indicator of power reserve remaining.
Some power reserve indicators are marked with a fixed number of days, while others show a simple “+/-” or “ab/auf” (“up/down” in German). There can also be power reserve indicators for further functions, for example the alarm function.
Breguet developed a prototype power reserve indicator in 1933 but did not bring it to production. Jaeger-LeCoultre introduced the modern power reserve indicator in 1948 on the Powerwind Ref. 163 watch's Cal. 481. Jaeger-LeCoultre heavily marketed this feature in the Futurematic watches of the 1950s and continues to offer a model bearing the name, Réserve de Marche today. Zodiac was another early company to use a power reserve indicator, introducing it in 1949 on a pioneering automatic watch movement.