An electronic music instrument which precedes the development of the synthesizer. The basic circuitry was devised by Russian expat Leon Theremin in 1920 (or 1919; sources disagree). The instrument caused a sensation at the time it was introduced, and Theremin received a job offer from RCA to develop a commercial version. However, the Great Depression killed the mass market, and the instrument has been somewhat of a specialist item ever since.
The theremin is unusual in that, in the process of playing, the performer does not touch it. The instrument has two metal rods or loops that protrude from its box; these are antennas attached to an oscillator that is running in the 100-200 KHz range. Moving a hand (or any other body part) alters the electric field around the antenna, which slightly alters the frequency of the oscillator. Within the box, the oscillator beats against a reference oscillator (in a process similar to amplitude modulation) to create an audio-frequency tone, which is separated from the RF frequencies and sent to an amplifier. There are two rods; by convention, the one which controls frequency extends vertically from the box, and the one which controls volume extends horizontally. Moving the hands near and around the antennas plays the instrument. There are no timbre controls, a significant limitation of the instrument.
Starting in the 1920s, Clara Rockmore started developing the technique for playing the theremin. She developed techniques for accurately intonating notes and playing without the excessive amounts of glissando that the instrument is usually noted for. She is regarded as the inventor of serious theremin playing technique.
Robert Moog, who is better known for his synthesizer designs, was a theremin enthusiast. It was his small theremin-manufacturing outfit which formed the basis for R. A. Moog, his first synthesizer company. He developed a version of the theremin which outputs MIDI data, and he produced a biographical movie about Rockmore.
Jimmy Page uses a Theremin in Led Zeppelin's Whole Lotta Love