metronome
A device that beats rhythm by audible and/or visible means, to help a musician maintain constant speed while playing. The first metronomes were likely pendulum-based and powered by wind-up clock action. Later models were powered by electric motor (e.g. Franz box metronomes, fed by AC). Pendulum types are still made and sold. However, most modern, self-contained metronomes are now electronic and battery-powered, unaffected by gravity or angle, with options for separate, mutable audio-visual cues and rhythmic sub-divisions.