Invented in 1919 by Leon Theremin (1896-1993)
Remains popular in experimental and electronic music circles today.
Developed in Russia, the first production model of the Theremin was introduced to the public at the Moscow Industrial Fair in 1920.
A few companies still manufacture Theremins, including Big Briar, owned by the late Robert Moog (of Moog synthesizer).
Its design included 2 metal antennae: a horizontal loop to control volume and a vertical rod to control pitch.
Instrument is played by moving one's hands in the air around the loop and the rod. The closer each hand gets to its antenna, the higher the pitch or the higher the volume.
The sound output is a continuous monophonic (one voice played at a time) tone that resembles a female voice or violin string sound.
An excellent listening example can be found on the Beach Boys song, "Good Vibrations."