When referring to the term Power Reserve, also known as Reserve de Marche, some people can become confused as to what the term actually means.
The actual term refers to the length of time a watch can run after it has been wound in its entirety. Every mechanical style watch has a very specific duration of time it will work before needing to be re-wound.
In order to power up the watch, a spring is wound very tight and carefully located within either a cylinder or a barrel. The tension from the spring releases slightly over the course of time, on a constant basis. The regularity of the spring releasing is what powers the watch over the course of time.
The Power Reserve is different from one watch to the next. Many factors play a role in this determination including whether it has been manually or automatically wound.
There is generally an indicator in some form that appears on the face of the watch. This will alert the wearer of the power reserve. Some come by marking days, hours or even a plus or minus sign to subtly demonstrate the need to add more power. However, some watchmakers may place an indicator on the back while others may not have any at all. There are automatic winders that can been attached to the watch to keep it powered properly when not being worn.
Manually wound watches are the ones that literally require you to wind them "by hand". Automatically wound watches contain a rotor that will advance while in use, but this only lasts a specific period of time. The average amount of time for an automatically wound timepiece is generally 38-42 hours, (however "by hand" can last for weeks).
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