====== Long power reserve ======
{{wst>image_cl|Jaeger-LeCoultre|Gyrotourbillon 1|Jaeger-LeCoultre Gyrotourbillon 1\\ with **8 days movement**}}

Watches with a **long [[power reserve]]** can continue to operate for longer than 2 days without additional winding.

Historically, the [[mainspring]] in a watch movement could keep it running for about 40 hours, or just under 2 days. This was a result of many factors 

  * [[Mainspring]]s must deliver consistent power
  * [[Barrel]]s must be small enough to fit inside a wrist-mounted movement
  * Winding tension must be low enough for hand or [[automatic]] winding mechanisms

As early as [[1889]], companies sporadically produced watches with longer power reserve. Some features massive [[barrel]]s containing very long springs, others used [[two barrel]]s or more, and some focused on increased movement efficiency. Many of these feature a [[power reserve indicator]] to show how much runtime remains. Note that some double barrel movements use the second barrel for other functions rather than longer power reserve.

Long power reserve watches are typically named for the number of days they can run without winding. **[[Hebdomas]]** ("Holy Week") watches feature a 7-day runtime, while others are simply named **5-day**, **7-day**, **8-day**, and so on.

===== History =====
The first 8-day pocket watch movement was invented in [[1888]] by Iréné Aubry and patented early the next year. Licensed by Arthur Graizely, it was produced and sold before the turn of the century under the names "Octava" and "Octodi", only acquiring the famous [[Hebdomas]] name in [[1906]]. Production was soon taken over by [[Schild & Co]] in [[La Chaux-de-Fonds]], and this company soon produced as many as 1,000 examples per day. It was later sold under the "Orator" brand, though "Hebdomas" was used as well in the 1960s and 1970s. After Schild became insolvent in [[1979]], production was restarted by [[Xantia]] in the 1980s and 1990s.

In [[1908]], [[Doxa]] introduced an 8-day watch movement that proved popular in World War I among soldiers.

[[Jaeger-LeCoultre]] introduced their first [[double barrel]] 8-day power reserve pocket watch in [[1919]] with [[JLC 144|Calibre 144]]. This was followed in [[1928]] with [[JLC 134|Calibre 134]], an 8-day [[alarm]] movement, and an 8-day [[minute repeater]] the next year. In [[1931]], Jaeger-LeCoultre introduced the most famous classic 8 day movement, [[JLC 124|Calibre 124]], which was used by many companies for decades. This basic design served as the template for [[JLC 879|Calibre 879]], introduced in [[1997]] in the [[Jaeger-LeCoultre Reverso Septantième|Reverso Septantième]] and later movements. Jaeger-LeCoultre also created the famous "stick" clock movement, which ran for 8 days.

In [[2000]], [[IWC]] revived their in-house movement capabilities with [[IWC 5000|Cal. 5000]], boasting 8-day power from a single mainspring, thanks to the high-torque [[Pellaton]] winding system. But this proved challenging to customers, since the varying power of this spring caused the watch to run fast at first, then slow as it wound down. IWC switched to a double-barrel system for the replacement [[IWC 52000|Cal. 52000 series]].

Some watches can control the varying force of the mainspring using a a special regulation system, as with the [[A. Lange & Söhne Lange 31|Lange 31]] or the [[DeWitt Academia Tourbillon Force Constante]], by which the driving force of the mainspring is held continuously equal during the entire running time.

The advent of [[silicon]] components has seen a revival of longer-reserve single-spring movements. By reducing internal friction in the [[escapement]] and other components, along with reducing the weight of the [[balance]], silicon movements have quickly advanced. For example, the [[2018]] [[Baume & Mercier]] [[BM12|Cal. BM12-1975A]] boasts 5 days of power reserve.

===== See Also =====
  * [[Power reserve]]
  * [[Power reserve indicator]]
  * [[Double barrel]]
  * [[Constant force]]


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