Jim Saunders asks about the Ondes Martinot which I believe is an
electronic keyboard instrument invented in the 30s.  It generates a
peculiar "electronic sound" with generates predominantly pure tones in
the fashion of another instrument of the time called the theromin.

The theromin generated a similar sound but was "played" by an operator
who held his hand/hands in the vicinity of a rod "antenna" which rose
from the small electronci box containing vacuum tube circuits.  The tone
was achieved by varying the distance from the operator and the antenna
which efectively varied the capacitance of an audio oscilator circuit which
changed the pitch continually rather than in steps like the ondes martinot.
The resulting sound was a continually varying sound characterised by an
eery continual pitch change effect frequently heard in science fiction
movies in the 30s and 40s.

Bernard Gregoire
Hingham, MA