The Rhythmicon—also known as the Polyrhythmophone—was the world's first electronic drum machine (or "rhythm machine", the original term for devices of the type).
In 1930, the avant-garde American composer and musical theorist Henry Cowell collaborated with Russian inventor Léon Theremin in designing and building the remarkably innovative Rhythmicon. Cowell wanted an instrument with which to play compositions involving multiple rhythmic patterns impossible for one person to perform simultaneously on acoustic keyboard or percussion instruments. The invention, completed by Theremin in 1931, can produce up to sixteen different rhythms—a periodic base rhythm on a selected fundamental pitch and fifteen progressively more rapid rhythms, each associated with one of the ascending notes of the fundamental pitch's overtone series. Like the overtone series itself, the rhythms follow an arithmetic progression, so that for every single beat of the fundamental, the first overtone (if played) beats twice, the second overtone beats three times, and so forth. Using the device's keyboard, each of the sixteen rhythms can be produced individually or in any combination. A seventeenth key permits optional syncopation. The instrument produces its percussion-like sound using a system, proposed by Cowell, that involves light being passed through radially indexed holes in a series of spinning "cogwheel" discs before arriving at electric photoreceptors.
Nicolas Slonimsky described its capabilities in 1933:
The rhythmicon can play triplets against quintuplets, or any other combination up to 16 notes in a group. The metrical index is associated ... with the corresponding frequence of vibrations.... Quintuplets are ... sounded on the fifth harmonic, nonuplets on the ninth harmonic, and so forth. A complete chord of sixteen notes presents sixteen rhythmical figures in sixteen harmonics within the range of four octaves. All sixteen notes coincide, with the beginning of each period, thus producing a synthetic harmonic series of tones.
Cowell had planned to exhibit the rhythmicon in Europe. In October 1931, in a letter to Ives from Berlin, he said, "I have been composing and have finished the second movement of my work for the Rhythmicon with orchestra for Nicolas to use in Paris in February."  Composer Charles Ives, Cowell's close friend, commissioned Theremin to build a second model of the Rhythmicon for use by Cowell and his associate, conductor Nicolas Slonimsky.
The Rhythmicon was publicly premiered January 19, 1932 by Cowell and fellow music educator and theorist Joseph Schillinger at the New School for Social Research in New York. Schillinger had known Theremin since the early 1920s and had a lifelong interest in technology and music.
The radically new instrument attracted considerable attention, and Cowell wrote a number of compositions for it, including Rhythmicana, 1931 (later renamed 'Concerto for Rhythmicon and Orchestra'), and Music for Violin and Rhythmicon (1932). Slonimsky said that Cowell's special piece Rhythmicana (presumably the one Cowell referred to in his letters to Ives) was completed too late to be used at the Paris concerts.
On May 15, 1932, a New Music Society concert in San Francisco included – along with the premiere of Xanadu, a new work by Mildred Couper – a demonstration of Cowell's new instrument. According to some sources, the concert premiered Cowell's "Rhythmicana", in four movements with orchestra, and "Music for Violin and Rhythmicon". According to several others, the Rhythmicana concerto was not performed publicly until 1971, and it was played on a computer. (Cowell later used the same title, Rhythmicana, for a set of solo piano pieces he composed in 1938.)
Before long the shine wore off.[clarification needed] In 1988, Slonimsky wrote:
Like many a futuristic contraption, the Rhythmicon was wonderful in every respect, except that it did not work. It was not until forty years later that an electronic instrument with similar specifications was constructed at Stanford University. It could do everything that Cowell and Theremin had wanted it to do and more, but it lacked the emotional quality essential to music. It sounded sterile, antiseptic, lifeless — like a robot with a synthetic voice.
Cowell soon left the Rhythmicon behind to pursue other interests and it was all but forgotten for many years.
One of the original instruments built by Theremin wound up at Stanford University; the other stayed with Slonimsky, from whom it later passed to Schillinger and then the Smithsonian Institution. This latter instrument is operational; its sound has been described as "percussive, almost drum-like". Theremin later (in early 1960s) built a third, more compact model after his return to the Soviet Union toward the end of the 1930s. This version of the instrument is operational and now resides at the Theremin Center in Moscow.
According to many unsubstantiated accounts, in the 1960s, innovative pop music producer Joe Meek experimented with the instrument, though it seems very unlikely that he had access to any of the original three devices; similarly, a number of accounts claim, without substantiation, that the Rhythmicon may be heard in the soundtracks of several movies, including Dr. Strangelove.
Schillinger once calculated that it would take 455 days, 2 hours, and 30 minutes to play all the combinations available on the Rhythmicon, assuming an average duration of 10 seconds for each combination.
- Albert Glinsky, Theremin: ether music and espionage. Urbana and Chicago: University of Illinois Press, 2000 p. 136. ISBN 0-252-02582-2.
- A similar but more sophisticated magneto-mechanical (rather than opto-mechanical) scheme would soon be used by Laurens Hammond to construct his first organ, introduced in 1935.
- Slonimsky, quoted in Leta E. Miller, Fredric Lieberm, Composing a world: Lou Harrison, musical wayfarer. University of Illinois Press, 2004, p. 12.
- "Rhythmicon". Oxford Music Online. Archived from the original on 2017-08-01.
- Mead, Rita H. (1981). Henry Cowell's New Music, 1925–1936. Ann Arbor, Mich.: UMI Research Press (excerpted online). ISBN 0-8357-1170-6
- Mooney, David R. (2007-09-21). "Opaque Melodies: The Rhythmicon: Background". Retrieved 2011-11-09.
- Glinsky, pp.140-1.
- ThereminVox.Com, Lev Sergeivitch Termen: The Inventor of the Theremin, p. 3.
"This apparatus, of which two models existed, one in Cowell’s possession and the other given up by Slonimsky to Schillinger - stirred great arousal in 1932 during an early concert at the New School for Social Research in New York."
- Shillinger eventually bought an original machine. Schillinger's widow donated the device to the Smithsonian Institution in 1966.(Mooney, op.cit.)
-  Greg Dixon, Turning Pitch Into Rhythm: Henry Cowell and the Evolution of the Rhythmicon. Perfect Sound Forever, October, 2009]
- ThereminVox.Com, op. cit.
" ... in 1932 ... Cowell wrote two compositions for it ... the concert was 'Rhythmicana' in four movements, a polyrhythmic percussive performance united with an orchestra and 'Music for Violin and Rhythmicon.' The first performance of this technical chamber music ... took place in the same year in San Francisco....". ThereminVox cites Fred K. Prieberg, Musica ex machina, Giulio Einaudi, 1963
- Madeleine Goss, Modern music-makers; contemporary American composers. Dutton, 1952, p.272.
- Barelos, Stacey. "Henry Cowell--Piano Music--Rhythmicana ". www.cowellpiano.com. Retrieved 31 May 2015.
- Barelos, Stacey. "Henry Cowell--Piano Music--Rhythmicana [Endnote 1]". www.cowellpiano.com. Retrieved 31 May 2015.
- 'A Life Story', quoted at Jim Horton, The History of Experimental Music in Northern California.
- Rhythmicon by Nick Didkovsky part of the composer's website. Retrieved 3/4/07.
- Schillinger, Joseph (1948). Mathematical Basis of the Arts (New York: Philosophical Library), pp. 666–667.
- Hicks, Michael (2002). Henry Cowell, Bohemian. Urbana and Chicago: University of Illinois Press. ISBN 0-252-02751-5.
- Lichtenwanger, William (1986). The Music of Henry Cowell: A Descriptive Catalogue. Brooklyn, N.Y.: Brooklyn College Institute for Studies in American Music. ISBN 0-914678-26-4.
- Nicolas Slonimsky, Electra Yourke, Perfect pitch: an autobiography. Schirmer Trade Books, 2002, 318 pp.
- Andrej Smirnov (demonstrator) (2005). "VIDEO: Rhythmicon demonstration (6 mb)" (m4u). Retrieved 2014-01-28. (1 minute 50 seconds video of Andrej Smirnov demonstrating a Rhythmicon with keyboard and spinning discs at the Theremin Center, Moscow, 2005)
- Andrej Smirnov (2005). Rhythmicon demonstration. Retrieved 2014-01-28. (Flash needed) (YouTube copy of Smirnov Rhythmicon demo)
- "The ‘Rhythmicon’ Henry Cowell & Leon Termen. USA, 1930" (at 120 Years of Electronic Music)
- The Schillinger Society
- American Mavericks: The Online Rhythmicon (Java applet)
- Rhythmicon for Windows