The ondes Martenot (French for "Martenot waves"; also known as the ondium Martenot, Martenot and ondes musicales) is an early electronic musical instrument with a keyboard and slide, invented in 1928 by Maurice Martenot - a French cellist, a radio telegrapher during the first World War, and an inventor. The instrument is originally very similar in sound to the Theremin. They both used the vacuum tube oscillator as a sound source and were both monophonic, where the Theremin had a sliding scale and no fixed preset notes the Ondes-Martenot had a keyboard and a strip control for glissando and vibrato and an appearance that was familiar to any keyboard player.

The instrument also had a bank of expression keys that allowed the player to change the timbre and character of the sounds. A later (1938) version of the instrument featured microtonal tuning as specified by the Hindu poet Rabindranath Tagore and the musician Alain Danielou.
The Ondes-Martenot was quickly accepted and eventually had a wide repetoire, works were written for the instrument by composers such as Edgard Varèse, Olivier Messian, Darius Milhaud, Arthur Honegger, Maurice Jarre.

[from http://weirdinstruments.blogspot.com]