Albert de Boer asked for information about the Ondes Martenot.
The following is from the book "Electronic Musical Instruments",
second edition (1958) by Richard H. Dorf.

"Les Ondes Martenot (literally The Martenot Waves) is the invention of
Maurice Martenot, a Frenchman.  It is manufactured under M. Martenot's
aegis in Neuilly-sur-Seine, probably in extremely limited quantities.
... The block diagram ... indicates the general functioning of the
Martenot.  It is a beat-frequency instrument with the usual fixed-
and variable-frequency oscillators.

            +--------+
            | Fixed  |
            | Osc.   |---
            |        |  |
            +--------+  |  +--------+  +---------+  +--------+   +-+/
                        +->|Detector|->|Amplifier|->|Controls|-->| |
Ribbon--+   +--------+  |  +--------+  +---------+  +--------+   +-+\
        |   |Variable|  |                                        Spkr
        +---| Osc.   |---
        |   |        |
  Keys--+   +--------+

"The variable oscillator is controlled in two ways.  The first is by
a seven-octave keyboard which switches in various capacitances.  The
second is by a long ribbon which runs the length of the keyboard and
over pulleys at the ends to operate some sort of variable capacitance.
To use the ribbon the player places his index finger in a sort of
thimble and moves it left and right to vary pitch and introduce
vibrato.

"The two oscillators are mixed in a detector which yields the audio beat
note, probably deliberately distorted to give some sort of complex
waveform.  It is amplified, then fed through several simple selectable
filter circuits, thence to a loudspeaker.  Others have the 'diffuseur-
resonateur' ... which consists of a transducer of some kind which
imparts vibrations to a sounding board with stretched strings.  The
strings are tuned to various pitches and when one or more is excited by
a particular output frequency of the instrument it vibrates and adds a
weird sort of lingering emphasis. ...

"The Martenot is an interesting novelty and has been characterized in
more glowing terms by at least one well known modern composer."

Regrettably, Dorf does not disclose who that modern composer is.

Paul West