The automatic watch is a masterpiece of mechanical convenience. But by what miracle do the movements of your wrist turn into power reserve, with no electrical source at all?

The secret lies in the oscillating weight (or rotor), a piece of heavy metal (often tungsten or gold) mounted on a ball bearing at the centre of the movement. When you move your arm, gravity sets the rotor spinning rapidly. The winding train then gears down this rotation to multiply the torque delivered to the mainspring, while reverser mechanisms wind it whichever direction the weight rotates in (bidirectional winding).

Two milestones shaped the modern technique: in 1931 Rolex introduced the Perpetual system, the first full 360° rotor (earlier solutions, such as John Harwood's 1923 bumper weight, oscillated between two stops); in 1948 Eterna refined the Eterna-matic by mounting the rotor on a ball bearing, a solution still in use today. In practice, eight to ten hours of daily wear is generally enough to keep the power reserve of a modern automatic calibre topped up.

Automatic winding is one of watchmaking students' favourite subjects, as it draws on a mechanism of remarkable visual ingenuity.

Rotors, reversers and ball bearings are dismantled on video in the online watchmaking course; if you are starting from zero, begin with watchmaking for beginners.

Want to understand how automatic winding systems work? Join Décalibre and discover in detail how the most famous automatic calibres function through our online modules.