In mechanical horology, a remontoire, (from the French remonter, meaning 'to wind') is a small secondary source of power, a weight or spring, which runs the timekeeping mechanism and is itself periodically rewound by the timepiece's main power source, such as a mainspring. In precision clocks and watches it is often used to place the source of power closer to the escapement thereby increasing the accuracy by evening out variations in drive force caused by unevenness of the friction in the geartrain. In spring-driven precision clocks a gravity remontoire is often used to replace the uneven force delivered by the mainspring running down by the more constant force of gravity acting on a weight. In turret clocks it serves to separate the large forces needed to drive the hands from the modest forces needed to drive the escapement which keeps the pendulum swinging. A remontoir should not be confused with a maintaining power spring, which is used only to keep the timepiece going while it is being wound.
Remontoires are used because the timekeeping mechanism in clocks and watches, the pendulum or balance wheel, is never isochronous; that is its rate is affected by changes in the drive force applied to it. In spring driven timepieces, the drive force declines as the mainspring runs down. In weight driven clocks the drive force, provided by a weight suspended by a cord, is more constant, but imperfections in the gear train and variations in lubrication also cause small variations. In turret clocks, the large hands, which are attached to the clock's wheel train, are exposed to the weather on the outside of the tower, so winds and accumulations of ice and snow apply disturbing forces to the hands, which are passed on to the wheel train.
With a remontoire, the only force applied to the clock's escapement is that of the remontoire's spring or weight, so that it is isolated from any variations in the main power source or wheel train, which is just used to rewind the remontoire. Remontoires are designed to rewind frequently, at intervals of 15 seconds to an hour. The rewinding process is triggered automatically when the remontoire's weight or spring reaches the end of its power. This frequent rewinding is another source of accuracy, because it averages out any variations in the clock's rate due to changes in the force of the remontoire itself. If the rate of the clock varies as the remontoir spring runs down, this variation will be repeated again and again, each time the remontoire goes through its cycle, so it will have no effect on the long term rate of the clock.
The gravity remontoire was invented by Swiss clockmaker Jost Burgi around 1595. Usually the "Kalenderuhr" (Three month running, springdriven, calendar-desk-clock) Burgi made for William IV, Landgrave of Hesse-Kassel (or Hesse-Cassel) (Now Inventory No. U 47 at the Naturwissenschaftlich-Technische Sammlung in Kassel) is considered the oldest surviving clock with a remontoire, even if it does not provide power to the escapement during the few seconds of the daily cycle where the remontoire weight gets wound up by the spring. Today remontoire mechanisms are all designed to deliver power to the escapement during the remontoire reset cycle.
Many French and Swiss pocketwatches after 1860 were stamped on the back with the word Remontoire. This merely meant that it didn't have to be wound with a key (i.e. they were wound by the then novel winding crown inside the pendent). Ethymologically the term is correct, the mainspring is "rewound" by some other force than a key, but these watches usually do not contain a remontoire as the word is used today.
They can also be classified by where in the wheel train the remontoire is located: