Complication guide
Power reserve indicator: how the complication works
A power reserve indicator shows how much of the wound mainspring is left, so the wearer knows how long the watch will keep running before it stops.
The mainspring is the watch's fuel tank, and a power reserve indicator is its fuel gauge. As the spring is wound it fills, and as the watch runs it empties; the indicator reads that state on a scale, often marked in hours, so the wearer can see at a glance how much running time is left.
Behind the dial a small differential gear train compares the winding input with the barrel's unwinding output and moves the indicator accordingly. It is most useful on a hand-wound watch, where there is no rotor to top the spring up, and on an automatic that has been resting off the wrist.
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