The Thing (listening device)

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.

The device was used by the Soviet Union to spy on the United States. It was embedded in a carved wooden plaque of the Great Seal of the United States. On August 4, 1945, several weeks before the end of World War II, a delegation from the Young Pioneer organization of the Soviet Union presented the bugged carving to U.S. Ambassador W. Averell Harriman, as a "gesture of friendship" to the USSR's war ally. It hung in the ambassador's Moscow residential study for seven years, until it was exposed in 1952 during the tenure of Ambassador George F. Kennan.

The Thing consisted of a tiny capacitive membrane connected to a small quarter-wavelength antenna; it had no power supply or active electronic components. The device, a passive cavity resonator, became active only when a radio signal of the correct frequency was sent to the device from an external transmitter. This is currently referred in NSA parlance as "illuminating" a passive device. Sound waves (from voices inside the ambassador's office) passed through the thin wood case, striking the membrane and causing it to vibrate. The movement of the membrane varied the capacitance "seen" by the antenna, which in turn modulated the radio waves that struck and were re-transmitted by the Thing. A receiver demodulated the signal so that sound picked up by the microphone could be heard, just as an ordinary radio receiver demodulates radio signals and outputs sound.

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