The power reserve indicator is one of the most useful complications of a mechanical watch, which shows the amount of remaining stored energy.
The power reserve indicator that displayed on the watch with manual-winding movement shows the time that left until the watch needs winding, while in automatic-winding watches it is used for determining time, during which a watch will function when it is not worn.
It is distinguished several types of power-reserve indicators by their appearance:
First power reserve indicators were used very early on marine chronometers and later in the Railroad pocket watches, but for wristwatches they were only created in 1933 by Breguet. However, this was only a prototype with only one watch assembled.
The mass production of power-reserve indicators for waristwatches was started by Jaeger LeCoultre manufactory in 1948th.
The principle of any power reserve device – either in manual or automatically winding watches – is based on a special train of gears that is connected to the ratchet wheel or the barrel arbor, which drives the power reserve indication showing the extent of the wind. The barrel’s teeth are also connected to the other side of this differential train so that as the mainspring's power is used, the power reserve indication moves in the opposite direction.