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. 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. Sound waves (from voices inside the ambassador's office) passed through the thin wood case, striking the membrane and causing it to vibrate. This is referred to in NSA parlance as "illuminating" a passive device. 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. The Thing consisted of a tiny capacitive membrane connected to a small quarter-wavelength antenna it had no power supply or active electronic components. Because it was passive, needing electromagnetic energy from an outside source to become energized and active, it is considered a predecessor of radio-frequency identification ( RFID) technology. Averell Harriman, the United States Ambassador to the Soviet Union, on August 4, 1945. It was concealed inside a gift given by the Soviet Union to W. 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. Covert listening device Replica of The Thing which contained a Soviet bugging device, on display at the NSA's National Cryptologic Museum
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