A power reserve indicator (originally called ) is a complication of the

watch A watch is a portable timepiece intended to be carried or worn by a person. It is designed to keep a consistent movement despite the motions caused by the person's activities. A wristwatch is designed to be worn around the wrist, attached ...

, which is designed to show the amount of remaining stored energy. The power reserve indicator indicates the tension on the

mainspring A mainspring is a spiral torsion spring of metal ribbon—commonly spring steel—used as a power source in mechanical watches, some clocks, and other clockwork mechanisms. ''Winding'' the timepiece, by turning a knob or key, stores energy ...

at any particular moment.

Overview

The power reserve indicator is one of the most useful features of a

mechanical watch A mechanical watch is a watch that uses a clockwork mechanism to measure the passage of time, as opposed to quartz watches which function using the vibration modes of a piezoelectric quartz tuning fork, or radio watches, which are quartz watches ...

besides the actual

time Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, t ...

display. A mechanical watch is operated by either automatic or manual winding. In order to run at a regular rate a mechanical timepiece needs to have at least 30 per cent of its mainspring wound. An automatic timepiece needs to be worn for about 10–15 hours before it is fully wound. The power reserve indicator displayed on the watch with automatic- winding movement shows how long a watch will function when not worn. On a manual winding watch, it shows the time left until the watch needs winding.

Brief history

There are numerous devices for recording the amount of mainspring power stored in the barrel. Power reserve indicators were employed very early on

marine chronometer A marine chronometer is a precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation. It is used to determine longitude by comparing Greenwich Mean Time (GMT), or in the mode ...

s and later in the accurate Railroad grade pocket watches. Today they are used in wrist watches. The first wristwatch with the mechanism was created by Breguet in 1933. However, this was only a prototype with only one watch assembled. In 1948, Jaeger-LeCoultre introduced a power reserve indicator designed for production in a series of watches called the Powermatic which utilized the LeCoultre Caliber 481. Thus, Jaeger-LeCoultre was ostensibly the first

watchmaking A watchmaker is an artisan who makes and repairs watches. Since a majority of watches are now factory-made, most modern watchmakers only repair watches. However, originally they were master craftsmen who built watches, including all their part ...

company to launch wristwatches with a power reserve indicator to the masses.

Functioning

In general, the indication of the power reserve is sometimes represented through a needle or hand or it may be indicted by a revolving wheel viewed through an aperture. Some more modern devices have linear indicators. In the case of a hand indicator, the needle or hand moves from one point of the indicator to another. The zone along which the needle moves, shows how much time is left until the mechanism would be expected to cease autonomous operation. Correspondingly, when the hand leaves the zone (or stops) the watch needs to be wound. The indicator itself represents the amount of tension of the mainspring, thus the lower the tension the less time left until another

winding An electromagnetic coil is an electrical conductor such as a wire in the shape of a coil (spiral or helix). Electromagnetic coils are used in electrical engineering, in applications where electric currents interact with magnetic fields, in de ...

of the watch is required.

The LeCoultre 481

The LeCoultre 481 utilized a differential device to show the power reserve stored in either a self-winding or hand-wound mainspring.

Operating Principle

The principle of any power reserve device is that when the mainspring is wound either automatically or manually, a special train of gears connected to the ratchet wheel or the barrel arbor 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.

Overview

Brief history

Functioning

The LeCoultre 481

Operating Principle

See also