Invented by Soviet scientist and spy Leon Theremin, the theremin is an electronic musical instrument. The sound is produced by the thereminist (performer) moving through electromagnetic fields to create an electronic tone. One hand is moved between two metal antennae and their relative position produces the oscillation of frequency. The second hand creates the amplitude or volume. The resulting, somewhat eerie sound, is played through a speaker.

The theremin has been used in the soundtracks of spooky, mystery thrillers, as the eerie sound is reflective of the genre. Musician, composer, performer and record producer Jean-Michel Jarre used the theremin in his concerts Oxygen and Space of Freedom.

More Info: en.m.wikipedia.org