This is a scientific instrument, based on the principles of beat frequency.In its early form, the volume was controlled by a foot pedal. But now it is changed by moving the hands near a metal loop. The theremin succeeded in attracting attention; in the 1930s RCA distributed it in the US. It was even used in several movie soundtracks and included in songs during the 1960s. One example is the song "Good Vibrations" by the Beach Boys.