How do you play an instrument without touching it?

You make your body part of the circuit. A theremin has two metal antennas, and each one forms half of a capacitor; your hand is the other half. Move your hand toward the upright pitch antenna and the note rises. Move your other hand away from the volume antenna and the sound gets louder. You never touch anything: you sculpt the air around the instrument.

Inside, two radio-frequency oscillators hum at almost the same frequency, far too high to hear. One stays fixed; the other shifts as your hand changes the capacitance. The difference between the two falls in the range of human hearing, and that difference is the note you hear. The instrument is absurdly sensitive: a well-designed circuit can shift a whole octave for a capacitance change of around 0.01 picofarads.

It was invented in 1920 by the Russian physicist Lev Termen, known in the West as Leon Theremin, out of Soviet research into proximity sensors. He later built something more sinister with the same knack for electronics: a listening bug hidden in a wooden Great Seal of the United States, which Soviet schoolchildren presented to the US ambassador in 1945. It eavesdropped on his Moscow office for seven years before anyone noticed.