What does the rotor in a self-winding watch do?

The rotor in a self-winding watch plays a crucial role in the mechanism of the watch by winding the mainspring automatically. As the wearer moves their wrist, the rotor—typically a semi-circular component—rotates due to the motion. This movement winds the mainspring, which is the power source for the watch. As a result, the energy stored in the mainspring enables the watch to operate without requiring manual winding. This automatic winding feature is what distinguishes self-winding watches from manual ones, making them convenient and efficient in daily use.

In contrast, the other options focus on different aspects of a watch's function. For instance, while movement is essential for timekeeping, the rotor itself does not directly keep the watch ticking; it provides energy to the mainspring, which then powers the movement. The rotor is also not responsible for charging a battery, as self-winding watches are mechanical and do not use batteries in the same way that quartz watches do. Additionally, although the rotor contributes to the overall functionality of the watch, it does not control the timekeeping function itself, which is managed by the escapement and balance wheel within the movement. Thus, the function of the rotor is specifically related to its role in winding the watch automatically