The Thing, also known as the Great Seal bug, was one of the first covert listening devices (or “bugs”) to use passive techniques to transmit an audio signal. It was concealed inside a gift given by the Soviets to the US Ambassador to Moscow on August 4, 1945. Because it was passive, being energized and activated by electromagnetic energy from an outside source, it is considered a predecessor of RFID technology.
The Thing was designed by Soviet Russian inventor Léon Theremin, whose best-known invention is the theremin, an electronic musical instrument.
The principal operational component of The Thing, a resonant cavity microphone, had been patented by Winfield R. Koch of the Radio Corporation of America (RCA) in 1941. In US patent 2,238,117 he describes the principle of a sound-modulated resonant cavity. High-frequency energy is inductively coupled to the cavity. The resonant frequency is varied by the change in capacitance resulting from the displacement of the acoustic diaphragm.