Power reserve: how long a watch runs off the wrist

The mainspring stores the energy that drives the movement. Wearing an automatic watch winds it through the rotor; a hand-wound watch is wound with the crown. The power reserve is how long the watch runs from fully wound until it stops.

Keeping an automatic running

• A day on the wrist usually keeps an automatic going, but a watch worn only briefly, or by someone who moves little, may not wind fully.
• To start a stopped automatic, give the crown a few dozen turns before setting the time. Most automatics can be wound by hand; a few, such as the Seiko 7S26, can only be wound by wearing or gently swinging them. Your watch's manual says which.
• A watch winder keeps a watch running between wears. It's a convenience rather than a necessity: letting a watch stop does it no harm.

Power reserve and accuracy

A movement keeps its steadiest rate when its mainspring is well wound. Near the end of the reserve the balance swings less widely, its amplitude falls, and the rate can drift. That's why a timegrapher reading should be taken fully wound, and why a watch left to run down every night can keep different time from one worn every day.

If a watch stops well before its rated reserve, or its amplitude stays low when fully wound, it may be due for a service.