On December 5, 1925, a Russian physicist named Lev Sergeyevich Termen — known in the West as Léon Theremin — filed a US patent application from New York for a device you could play by waving your hands in empty air. On February 28, 1928, the USPTO granted him US Patent 1,661,058, "Method of and Apparatus for the Generation of Sounds." It was the first patent for an electronic musical instrument, and the first patent for touchless human-machine interaction.

The trick was capacitance. Theremin's device had two antennas — a vertical rod for pitch, a horizontal loop for volume. Each antenna formed one plate of a capacitor; the player's hand formed the other. Move your hand closer, the capacitance rises; move it away, it falls. Inside the cabinet, two radio-frequency oscillators ran at nearly the same frequency, and the hand-antenna capacitor pulled one of them slightly. The circuit subtracted the two RF signals — a technique called heterodyning — and the audible difference frequency drove a loudspeaker. Wave your hand, change the pitch. No keys, no strings, no contact.

Theremin had built the prototype in 1920 at Petrograd's Physico-Technical Institute while researching gas dielectrics. He demonstrated it to Lenin in 1922 (Lenin reportedly played a passable "Skylark"), toured Europe and America with it, and licensed manufacturing to RCA. Then in 1938 Soviet agents pulled him back to Moscow, where he spent years in a sharashka — a prison laboratory — inventing surveillance gear for the NKVD. His most famous prison invention was "The Thing": a passive resonant cavity, no battery, no active electronics, hidden inside a carved wooden Great Seal that Soviet schoolchildren gifted to the US Ambassador in 1945. When a Soviet transmitter aimed a microwave beam at the embassy, the cavity re-radiated the ambassador's voice back. It went undetected until 1952.

Both inventions have modern descendants everywhere:

• Capacitive touchscreens. The screen you're reading this on is a grid of tiny capacitors. Your finger — a lossy conductor at ground potential — perturbs the local field exactly the way Theremin's pitch antenna sensed his hand. Apple's 2007 iPhone patents cite decades of intermediate capacitive-sensing work, but the fundamental physics is Theremin's.
• Proximity and gesture sensing. Every phone that dims its screen when you hold it to your ear, every laptop trackpad, every elevator button that lights up before you touch it — all use capacitive coupling to a human body. Google's Project Soli radar and automotive "kick to open" trunks are direct spiritual descendants of pitch-antenna control.
• Heterodyne detection. The RF beat-frequency trick that turned tiny capacitance changes into audible tones is the same principle used in every superheterodyne radio, every FM tuner, and every software-defined radio front end.
• Passive RFID and backscatter. "The Thing" — a resonant cavity that modulated a reflected illuminating signal — is the direct ancestor of passive RFID tags, battery-free sensors, and ambient backscatter Wi-Fi. Every contactless credit card owes something to Theremin's prison workshop.

The patent itself is short and startlingly readable: Theremin claims a musical instrument "controlled solely by the movement of the player's hand in space." Ninety-eight years later, that sentence describes not just a niche instrument played by Clara Rockmore and used in Star Trek theme music, but the primary input modality of the pocket computer in nearly every human hand on Earth.