Mechanical Principles › Complications › Other Complications

THE POWER RESERVE DISPLAY

The power reserve represents the autonomy of a mechanical watch, that is, the duration during which it runs between the full winding of the mainspring and its stopping, for lack of energy. It is generally expressed in hours and depends on the length and characteristics of the mainspring, the number of barrels, and the efficiency of the kinematic chain. A power reserve of forty to eighty hours is common; some constructions exceed several days. On an automatic watch, the oscillating weight (rotor) continuously rewinds the barrel with each movement of the wrist, maintaining the reserve. Many timepieces display the state of this autonomy thanks to a power reserve indicator, allowing the wearer to know when to wind the watch. Power reserve is both a technical and practical criterion: it reflects the energy efficiency of the movement and prevents unexpected stoppages that would require resetting the watch’s time.