In the early 1930s, maverick composer Henry Cowell collaborated with inventor Leon Theremin to build an electronic instrument capable of producing intricate polyrhythms. This instrument, dubbed the Rhythmicon, can be considered a rudimentary example of an interactive music system. Cowell and Theremin created the machine to fulfil a compositional need, but it ultimately failed to become a successful musical instrument. The Rhythmicon was one of the first electronic music instruments to use technology to extend performers' musical capacities, anticipating the interactive computer music movement by several decades. Despite its shortcomings, the Rhythmicon should be remembered as an important step on the road to interactivity.