The barrel is the round metal drum that holds the mainspring in a mechanical watch. It has a lid and gear teeth around its rim. This is where the watch gets its power. With the tensioned spring inside, the toothed barrel works as the first wheel of the gear train.
How does a watch barrel work?
The mainspring sits coiled inside the drum. Its outer end hooks onto the barrel wall. Its inner end hooks onto the central arbor, the axle at the middle of the barrel. As the spring unwinds, it turns the barrel. The teeth around the rim mesh with the first pinion of the wheel train, which is usually the centre wheel. From there, power passes through the rest of the movement. The barrel itself turns slowly. It usually makes one full turn about every eight hours.
What is a going barrel?
The going barrel is the type used in modern watches. The French watchmaker Jean-Antoine Lépine invented it in 1760. It is wound by turning the arbor, and it drives the movement through its teeth. Older fusee watches worked differently. They used a plain barrel with no teeth. A chain or cord wound around it linked it to the fusee. Two related designs also exist. A hanging barrel is a going barrel that saves space, because the movement supports it only at its top end. The motor barrel, used in pocket watches around 1900, reverses the going barrel’s layout.
How is the barrel wound, and does its size matter?
In an automatic watch, the moving rotor winds the barrel. In a manual watch, the wearer winds it by turning the crown. Either way, winding puts tension back into the mainspring inside the drum. Size counts too. A bigger barrel can fit a longer mainspring. A longer mainspring gives the watch a longer power reserve, so it runs for longer between windings.
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