In the light of a recent comment, I’d like to write something about one of the major controls of an Ondes Martenot: the touche d’expression.

As the instrument produces a continuous tone of variable pitch, there’s an urgent need for some kind of volume control.
Almost like the common volume potentiometer in any audio amplifier, the touche d’expression controls the output level by limiting the cathode current of the output tube (EL84) depending on the pressure applied to the white button (see the photo below). Instead of a linear or circular carbon film in a common potentiometer, the Ondes Martenot uses a small leather bag filled with carbon powder, similar to the microphones used for telephones prior to the electronically amplified microphones that came up in the 1980s.
For some thoughts about the contens of the bag, see this discussion on radiomuseum.org (in german language)

This is a photo of the touche d’expression from a 1961 Ondes Martenot still needing to be refurbished. Using the pretension thread, some pressure is applied to the carbon powder just enough to keep the output silent. The 470k resistor tied to plate voltage lifts the cathode potential of the EL84 up some volts to allow for complete silence with the pretension adjusted properly and optimum response to the pressure applied by the player.

A second unit of this kind is shown on the right side of the schematics above. This way some low pass filtering is implemented, controllable with a knee pedal under the instrument.