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The Thing , also known as the Great Seal Bug , was one of the first eavesdroppers (or "bugs") to use passive techniques to transmit an audio signal. By the Soviet Union on August 4, 1945, the United States Ambassador to the Soviet Union  It was hidden inside a gift given to W. Averell Harriman.

The Thing was conceived by Soviet Russian inventor Léon Theremin , who is credited with inventing the theremin, an electronic musical instrument.

Embedded in a carved wooden plate of the Great Seal of the United States, the device was used by the Soviets to spy on the United States.

On August 4, 1945, a few weeks before the end of World War II, a delegation from the Soviet Union's Young Pioneer organization detected the secretly engraved bugging device as an "act of friendship" to the Soviet Union's war ally, Ambassador Harriman. presented a

It hung in the ambassador's residential study in Moscow for seven years, until it was revealed in 1952 during Ambassador George F. Kennan's tenure.

Working Principle

Well, it consisted of a small capacitive membrane attached to a small quarter wavelength antenna; It had no power supply or active electronics.

A passive cavity resonator, the device would only become active when a radio signal of the correct frequency was sent to the device from an external transmitter.  This was referred to in NSA language as "lighting up" a passive device.

Sound waves (from the sounds inside the ambassador's office) pass through the thin wooden case, hitting the membrane and causing it to vibrate.

The movement of the membrane changed the capacitance "seen" by the antenna, which modulated the radio waves that were multiplied and retransmitted by The Thing. Just as an ordinary radio receiver demodulates radio signals and outputs sound, a receiver demodulated the signal so that the sound received by the microphone could be heard.


The Theremin's design made the bugging device very difficult to detect because it was so small, had no power supply or active electronics, and did not emit any signals unless actively teleported from afar. These same design features, together with the overall simplicity of the device, made it very reliable and gave it a potentially unlimited working life.



listening device  Its existence was discovered by chance in 1951 by a British radio operator at the British embassy, who overheard American conversations on the open radio Soviet air force radio traffic channel while the Soviets were emitting radio waves in the ambassador's office. A US State Department employee was then able to reproduce the results using an untuned broadband receiver with a simple diode detector/demodulator, similar to some field strength meters.

In March 1951, two additional employees of the State Department, John W. Ford and Joseph Bezjian, were sent to Moscow to investigate this and other suspected insects in the British and Canadian embassy buildings. They conducted technical analysis and bug detection of the ambassador's office against technical surveillance, using a signal generator and a receiver in a setup that produces audio feedback ("hum") if sound from the room is transmitted at a certain frequency.  During this listening device search, Bezjian found the device in the Great Seal carving.

The Federal Bureau of Investigation set out to analyze the device and recruited people from the British Marconi Company to assist with the analysis.  The research was conducted by British scientist and later MI5 counterintelligence officer, Marconi technician Peter Wright. He was able to reliably operate the Thing with an illuminating frequency of 800 MHz. The generator that discovered the device was set to 1800 MHz.  The Thing's membrane was extremely thin and was damaged during use by the Americans; Wright had to replace it.

The simplicity of the device caused some initial confusion during its analysis; The antenna and resonator had several resonant frequencies in addition to the main frequency, and the modulation was partially both amplitude modulated and frequency modulated.

The team also wasted some time assuming that the distance between the membrane and the tuning post had to be increased to increase resonance.

There were later models of the device, some with more complex internal structure (the center post under the membrane was connected to a helix, presumably to increase Q). Maximizing the Q-factor was one of the engineering priorities, as this allowed higher selectivity on the illumination signal frequency and therefore a higher working distance, as well as higher acoustic sensitivity.

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