Composed of a drum with teeth around it, a shaft in its center, a spring inside and a cover closing it all, the barrel is the energy accumulator of a mechanical watch.

It can be wound by hand using the crown or by gestures when its movement is equipped with an automatic system.

Barrel springs of automatic watches are all equipped with a slipping spring that will allow the spring to slide when the spring is fully wound. This will prevent the oscillating weight from breaking the barrel spring at that time. Thus, when you wind a watch and the winding process stops at the end, you have confirmation that it is a manual winding movement. If it were automatic, the slip-spring would not mark this stop but would slide indefinitely.

In skeleton watches, when the barrel is openworked, the position of the spring coils can be interpreted almost as an indicator of power reserve. When they are on the outside, the watch tends to stop and on the contrary, when they are tightened towards the inside, the barrel is at the maximum of its accumulation. A barrel has about seven revolutions of development.

Historically, blued steel springs had a force that went down regularly, unlike the Nivarox springs made of white metal developed in the 20th century, whose force is almost constant from the beginning to the end.

To compensate for the change in force of the blued spring barrels, the movements were sometimes equipped with a mechanism with a maltese cross which isolated the five best turns of armage. Thus the first one where the force was sometimes too strong and could create a defect called the knock was isolated just like the last turn which sometimes was too weak. Thus, the barrels with a Maltese cross had a power reserve reduced by about a third but of better quality.

Also, to maintain the strength of the constant blued springs some watchmakers have brought the fusee. A mechanism that weakens the force when it is too strong and strengthens it when it is too weak.

It is interesting to mention that this fusee mechanism reappeared on wristwatches at the end of the 20th century. At that time, when almost all the barrels were equipped with white metal Nivarox springs, it was necessary to remake blued steel springs because the fuses work with blued steel springs. A fuse on white metal Nivarox spring would have created defects on this spring by creating rebat for example. Apart from their colors, blue steel springs or white metal Nivarox springs can be recognized by their shape after being clamped once. The blue steel springs, once they have been retracted once, will remain circular in shape while the white metal Nivarox springs will always keep their initial “S” shape when they are out of the barrel.