There were two people we know of who were working on the invention of a heterodyne musical instrument in the years immediately following World War I: Lev Termen and Maurice Martenot.
In the early days of the prototypes that were to eventually develop into the theremin and the ondes, the differences between the two instruments were not so clear cut.
The earliest theremin prototype had no volume antenna (volume was controlled by a foot pedal) and the earliest ondes had no keyboard (pitch was played "a distance" by hand gestures).
There was no contact between Termen and Martenot prior to the presentation of their respective instruments in 1927 at the Paris opera. The conductor Leopold Stokowski attempted to get the two of them together in the 1930's when Martenot visited America but, not surprisingly, nothing came of the encounter. The two men were competitors, not colleagues!
Like most inventions, the theremin and the ondes did not just appear miraculously out of nowhere. They were the natural result of the coming together of a number of earlier discoveries, the most important of which was probably the radion tube.
The ondes Martenot has largely disappeared. It is extremely expensive to make and it has been replaced by the far more versatile (not to mention polyphonic) modern keyboard synthesizer.
The theremin has survived because it is comparatively cheap to manufacture and it has tremendous appeal for the musical hobbyist and gadgeteer. It has never attracted significant numbers of professional career musicians because you cannot make a living playing it. Learning to master the theremin requires a tremendous investment of time and energy and it is limited in regard to what can be performed on it.
The theremin is a one-trick pony but the trick, when it is successfully pulled off, is stunning!
