In March last year I wrote something about the Ether 2005 Festival and the evening with Jonny Greenwood and Thom Yorke. In that posting:
Ondes Martenot: what a world is here! Much information on the web, so for starters only: obsolete.com (the Keyboard Museum); Wikipedia; Claude-Samuel Levine's ondes Martenot site; Christine Ott (contemporary ondes Martenot artist); Sound on Sound (interesting article about the instrument, the Cornish company, Analogue Systems, and their 'French Connection' "version" of the ondes Martenot, as commissioned by Jonny Greenwood).
Now, via John Coulthart, I've come across Peter Pringle on the ondes Martenot ('explores - mostly through archival photos - the simultaneous development and presentation of the ondes Martenot and the theremin by their respective inventors: Maurice Martenot and Leon Theremin'):
The theremin has a "sister instrument" whose construction was based on the same electronic, "heterodyne" principles as those used by Leon Theremin. Conceived and designed by the French cellist and inventor, Maurice Martenot 1898 - 1980 … he built his first instrument at roughly the same time as Leon Theremin was working on his own prototypes. He called his invention the "ondes Martenot" … There was no contact whatever between Maurice Martenot and Leon Theremin until they were introduced in New York City in 1930. Their inventions were totally independent of one another and, by 1930, both men had already introduced their respective instruments to the world.
There are, indeed, some wonderful photographs ('I am indebted to Maurice Martenot's biographer, Jean Laurendeau') on the three pages Peter Pringle has put together, one of his own favourites being this one (click through to visit this, page 3 of his ondes Martenot entry) — 'taken at the World's Fair in Paris, 1937 (at which Maurice Martenot was awarded "Le Grand Prix de l'Exposition Mondiale"). The ensemble consists of eight ondes Martenots, a percussionist and a pianist, and is conducted by Ginette Martenot, sister of Maurice':

There's 'a short mp3 sample of ondes virtuoso, Jean Laurendeau, playing the Concerto for Ondes Martenot and Orchestra by Jacques Hétu' here (mp3).
Peter Pringle also has a page on the theremin cello, 'one of the five musical instruments invented in the early 20th century by Leon Theremin (the other four being the theremin, the theremin keyboard, the rhythmicon and the terpsitone)'.

