Metronome, an instrument for measuring time in music. It is a kind of pendulum whose centre of oscillation is beyond the point of suspension, contrived so that it may be easily carried about and placed conveniently on a table. The first metronomes went without clockwork, consisting simply of a rod with sliding balls at either end, and suspended near the middle on a horizontal rod which served as an axle. (See Mechanics.) The modern instrument is kept in motion by clockwork, and usually consists of a wooden pyramidal box on the front of which is a graduated scale of figures numbering from above downward. A pendulum, bearing a sliding weight a and a bob b, has its rod graduated with marks corresponding to those on the scale. As the sliding weight is moved up, the centre of oscillation is moved further beyond the point of suspension c, and the vibrations take place more slowly. The scale in the instrument from which the drawing was made has a range from 40 to 208, the numbers corresponding to the number of beats per minute, which is the unit of time. In some instruments a bell has been added, arranged so as to strike at the beginning of each bar. The knob shown at d moves the bell by means of a slide; e is the key for sounding.
Modern composers are in the habit of marking their compositions with the metronomic signs, and many of the principal works of the older composers have recently been thus marked by the editors. These signs consist of a note together with its numerical or metronomic value. For example, if a movement is marked ρ=132, that implies that when the sliding weight is set at 132 on the scale the pendulum will vibrate once to each quarter note in the bar. Similarly, ρ = 80 would signify that when adjusted at 80 the pendulum would vibrate once to each half note. The credit of this invention is usually given to Maelzel, but it more properly belongs to Diederich Winkel of Amsterdam, who made the first instrument about 1815. Maelzel improved upon it somewhat, and appropriated the invention.