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(n.) Possibly the first electronic musical instrument, invented in 1919 by the Russian Lev Sergeivitch Termen (which he later changed to Leon Theremin). It is the only musical instrument that is played without being touched. A theremin works by causing two oscillators to “beat” together. The beat frequency equals the difference in frequency between the two signals. Beats are a physical phenomenon occurring in the air when sounds are mixed. A theremin uses one oscillator operating well above the upper limit of human hearing as a reference tone, and another oscillator whose frequency is varied by the proximity of a human hand, for instance, to a capacitive sensing element shaped like an antenna. A typical machine has two antennas and you play it by moving your hands nearer to and farther from the antennas. One antenna controls the volume of the sound, while the other controls the frequency, or pitch, of the sound.