Maurice Martenot - ondes Martenot 1928
Maurice Martenot - ondes Martenot 1928

Maurice Martenot created the first Ondes Martenot, based off his findings of pure vibrations in the first world war.The instrument is monophonic and consists in three diffusers : a principal diffuser (a traditional loudspeaker),a resonnance diffuser (two possibilities : one originally named palme and the metallique (a gong set in vibration by an engine, the metal replacing the membrane of the loudspeaker, which creates a metal sound halo at a precise pitch).In front of and along the keyboard there is a ruban (ribbon) with a ring through which the interpreter puts the forefinger of his right hand. The frequency corresponds to the key facing the ringOn the left, a drawer contains all the controls : sounds, transposition buttons , loudspeakers controls, balance, pink noise and intensity key. As with a bow, no sound is produced if the interpreter does not press that key which allows to create intensities and attacks. Two foot-controlled pedals complete the instrument.external image 330px-Ondes_martenot.jpg
Here's 2 videos that shows and explains the instrument well

Source
http://www.thomasbloch.net/en_ondes-martenot.html