The Ondes-Martenot was invented in 1928 by Maurice Martenot. Originally the sound was similar to that of the theremin, but by adding timbral controls and switchable loudspeakers the range of sounds was expanded. Sounds are created by varying the frequency of oscillation in vacuum tubes.
This instrument sounds very much like the musical saw. It was used by Jacques Brel (Ne me quitte pas) and the Beach Boys (Good Vibrations).
Source: http://cafemusique.wordpress.com/2010/11/11/les-grandes-ondes/
http://en.wikipedia.org/wiki/Ondes_Martenot
http://www.youtube.com/watch?v=4wowFxcDrXA