The Theremin is an early electronic musical instrument, invented in Russia in 1928 by Léon Theremin. It’s controlled by two antennas which sense the location of the player’s hands, with one being used to control the wave frequency, i.e. pitch and the other being used to control the wave amplitude, i.e. volume. It’s supposed to be very easy to learn but very difficult to master, especially because there are no frets or keys to guide your hands.
In the video, Clara Rockmore plays Wieniawski’s “Romance”. Ms. Rockmore was one of the first performers to bring musical artistry to the theremin. You may recognize the sound from old Sci-Fi movies.