Metronomes are devices that are used by musicians to help them play at a specific tempo without any variation. The metronome was invented in 1816 and within just a few years every serious musician owned one, whether as large as a grandfather clock or small enough to carry in a pocket. Beethoven was one of the first composers to take advantage of the existence of the metronome by indicating very specific tempos on his compositions based on metronome settings, such as "quarter note equals 140," which means that a metronome set at 140 beats per minute will click on every quarter note. Today many musicians prefer electronic metronomes.
Clockwork metronomes consist of a double pendulum, a weight, and a casing, inside of which is a spring, a fixed bar, and a lever. The casing contains a fixed bar to which the pendulum is attached, usually near the middle of the pendulum. The pendulum is notched with settings, and the weight can be slid up and down the entire pendulum between these notches to control how fast the double pendulum swings. Settings are based on beats per minute, so a setting of 40 means that the metronome will tick 40 times per minute. When the metronome is set in motion, it falls to one side because of gravity. The internal spring pushes it back to the other side, and so the pendulum keeps moving back and forth. When the pendulum reaches the bottom of its swing, it presses a small lever that causes it to click.
Electronic metronomes have been around since the 1950s and use batteries. They have a circuit board inside that controls the speed and volume of the beep and the flash of the LED light on the outside produced when the metronome is turned on. Often electric metronomes also function as tuning machines, playing an A through a small speaker.