There's one friend of every musician that never gets true acknowledgement for the helping hand it gives: the humble metronome. In 1815 this clever little device was patented by Johann Maelzel under the title Instrument/Machine for the Improvement of all Musical Performances, called Metronome. Today, this ingenious device helps maintain a constant tempo, and find out what that tempo should be according to the composer's wishes.
The metronome is not without controversy, however, since its rigid, relentless pulse is often felt to be prohibitive in achieving a truly musical effect that would include accelerando and rallentando according to the performer's sense of the phrasing. In 1880 a book on the art of music called it "a lifeless, soulless machine", whilst a decade before Franz Liszt had called a metronomical performance "tiresome and nonsensical".
It was Ludwig van Beethoven who both took up the use of metronome markings before other composers and who also understood how this invention could be used for good. "100 according to Maelzel", he wrote as a marking on one composition, "but this must be held applicable to only the first measures, for feeling also has its tempo and this cannot entirely be expressed in this figure". Beethoven added metronome markings to all of his first eight symphonies following its invention, although modern scholars believe that his metronome was quite inaccurate since his suggestions would make these works ludicrously fast!