An automatic watch (also called a self-winding watch) is a mechanical watch, typically with a balance wheel escapement, whose mainspring is wound by the motion of the wearer's arm, instead of having to be wound manually every day. Whereas a quartz watch is powered by electricity, a mechanical watch is powered by a mainspring which must be rewound for the watch to keep time.

The name comes from the point that, instead of the owner having to wind the watch to power, the watch winds itself "automatically" when worn regularly. To accomplish this, the watch contains a fan-shaped rotor (weight) within the watch case. The normal movements of the user's arm and wrist cause the rotor to pivot back-and-forth on its staff and the staff is attached to a ratcheting winding mechanism inside the watch. Movement of the user's arm is thereby translated into circular motion of the rotor that eventually winds the mainspring.

On some automatic watches the rotor is visible through a transparent case back, called a display back or exhibition back. In these cases the rotors are often engraved or decorated in some way.

For those who don't wear their watches every day, watch winders are available to store automatic watches. A watch winder is a device that can hold one or more watches and moves them in circular patterns to mimic the human motion that keeps the self-winding mechanism working.

A more recent evolution of the automatic watch uses a self-winding mechanism to charge a battery or capacitor which in turn powers a quartz movement. This automatic quartz arrangement provides the accuracy of quartz without the need to replace the battery or capacitor until it reaches the end of its life, which may be decades.

History

The Swiss watchmaker Abraham-Louis Perrelet invented a self-winding mechanism in 1770 for fob watches. It worked on the same principle as a modern pedometer, and was designed to wind as the owner walked, using an oscillating weight inside the large watch that moved up and down. The Geneva Society of Arts reported in 1776 that fifteen minutes walking was necessary to wind the watch sufficiently for eight days, and the following year reported that it was selling well.^*

Perrelet sold some of his watches to a contemporary watchmaker, Abraham-Louis Breguet who improved the mechanism in his own version of the design; calling his watches "perpetuelles" (possibly the source for Rolex's brand name of the "Perpetual" watch brand).^* Timekeeping in Europe and China : Watches & Wonders timeline at Worldtempus.com

The mechanism was more successful on wristwatches because the rotor could operate every time that the owner moved their arm. However the first version did not appear until the 20th century. It was invented by a watch repairer from the Isle of Man named John Harwood in 1923Press release by UK Patent Office on 80th anniversary of self-winding watch patent, who took out a UK patent with his financial backer, Harry Cutts, on 7 July 1923, and the Swiss patent on 16 October 1923.Brainy History - 16 October 1923 It used a pivoting weight which swung as the wearer moved, and which in turn wound the mainspring. When fully wound, the watch would run for 12 hours if the wearer removed it. It did not have a conventional stem winder, so the hands could be moved manually by rotating a bezel around the face of the watch.

The watches went on sale in 1928 and 30,000 were made until the company collapsed in 1931 as a result of the Great Depression. The Rolex Watch Company had taken up the design in 1930 and used it as the basis for the Rolex Oyster Perpetual, in which the centrally mounted semi-circular weight could rotate through a full 360 degrees rather than the 300 degrees of the original design. Rolex's version also increased the amount of energy stored in the mainspring, allowing it to run for up to 35 hours if the wearer removed the watch.

Virtually all mechanical watches sold today are automatic. A notable exception is the Omega Speedmaster Professional "Moonwatch", the model used by NASA astronauts during the Apollo Program. (The weightless environment meant that automatic watches were not considered reliable for space travel).

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