"Tempoi parendum. [One should be compliant with the times.]"
Maxim of Theodosius II
The tempo of a piece of music, that is, the number of beats per minute, can be established using a metronome, a type of compact, adjustable, loud clock. The most common type is powered by simple clockwork and has a vertical metal rod that swings from side to side making a loud clicking sound at every swing. The rate of swing can be adjusted by moving a small weight up or down the swinging bar. Up decreases the tempo, and down increases it. This helps musicians not only establish the intended beat, but also maintain it throughout a musical piece.
The first metronome was made in 1696 by the Parisian Etienne Loulie (1654-1702). This required a single-weighted pendulum, similar to that of a grandfather clock. It had no clock escapement to maintain the pendulum in motion, so it only gave the musician the beat for a limited time. In 1812 Dietrich Nikolaus Winkel invented a version in Amsterdam. His breakthrough was the realization that a short, 8-inch (20 cm) metal pendulum, weighted both above and below the pivot, could be made to sound out a low tempo of forty to sixty beats per minute. Johann Malzel patented the well-known small portable metronome in 1816 using Winkel's basic design. The first composer to mark his music with the expected metronome-regulated tempo was Ludwig van Beethoven, in around 1817.
Needless to say, electronic metronomes now vie with the mechanical version. These include sophistications such as additional sounds, and can sound out complicated time signatures, such as 5/4, that are beyond the range of their predecessors.
