Mainspring

Definition

The coiled ribbon of steel that stores a mechanical watch's energy, wound by the crown or by the rotor of an automatic and housed in a drum called the barrel. Its length and the movement's appetite set the power reserve. A spring does not deliver power evenly: it pushes hardest fully wound and weakest near the end, which is why some makers stop the movement before the barrel is truly empty, and why a second barrel is the usual way to a longer reserve. Omega's 8500 and Panerai's P.9010 use two; IWC's seven-day 51111 uses one very large one; Panerai's P.2003 uses three for ten days.