A barrel is a cylindrical component in a watch that houses the mainspring, providing the energy needed to power the movement.
The barrel is a cylindrical container housing the mainspring—the primary energy source of a mechanical watch. As the mainspring is wound, it stores tension inside the barrel. When released, this energy flows through the gear train to power the movement. The barrel's diameter and spring length determine the watch's power reserve.
Larger barrels accommodate longer, thicker mainsprings that store more energy, resulting in extended power reserves. Most single-barrel watches achieve 40-70 hours of reserve. Some movements use multiple barrels connected in series to dramatically extend power reserve—movements with 3-4 barrels can achieve 8-10 days of running time.
A going barrel directly drives the gear train as the mainspring unwinds, which is simpler but delivers variable torque as tension decreases. A fusee and chain system compensates for this uneven force using a cone-shaped pulley. Modern mainspring alloys deliver sufficiently consistent force that fusees are largely obsolete except in high-end traditional pieces.