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 Soviet Union to W. Averell Harriman, the United States Ambassador to the Soviet Union, on August 4, 1945. 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.

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 referred to in National Security Agency (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.

Its design made the listening device very difficult to detect, because it was very small, had no power supply or active electronic components, and did not radiate any signal unless it was actively being irradiated remotely. These same design features, along with the overall simplicity of the device, made it very reliable and gave it a potentially unlimited operational life.

The device consisted of a 9-inch-long (23 cm) monopole antenna (quarter-wave for 330 megahertz [MHz] frequencies, but it was also able to act as half-wave [at 660 MHz] or full-wave [at 1320 MHz]; the accounts differ. Given the radio technology of the time, the frequency of 330 MHz is most likely, equivalent to a wavelength of 91 cm. It used a straight rod, led through an insulating bushing into a cavity, where it was terminated with a round disc that formed one plate of a capacitor. The cavity was a high-Q round silver-plated copper "can", with the internal diameter of 31⁄40 in (19.7 mm) and about 11⁄16 in (17.5 mm) long, with inductance of about 10 nanohenries. Its front side was closed with a very thin (3 thou, or 75 micrometers) and fragile conductive membrane. In the middle of the cavity was a mushroom-shaped flat-faced tuning post, with its top adjustable to make it possible to set the membrane-post distance; the membrane and the post formed a variable capacitor acting as a condenser microphone and providing amplitude modulation (AM), with parasitic frequency modulation (FM) for the re-radiated signal. The post had machined grooves and radial lines into its face, probably to provide channels for air flow to reduce pneumatic damping of the membrane. The antenna was capacitively coupled to the post via its disc-shaped end. The total weight of the unit, including the antenna, was 1.1 ounces (31 grams).

The length of the antenna and the dimensions of the cavity were engineered in order to make the re-broadcast signal a higher harmonic of the illuminating frequency.

The original device was located with the can under the beak of the eagle on the Great Seal presented to W. Averell Harriman (see below); accounts differ on whether holes were drilled into the beak to allow sound waves to reach the membrane. Other sources say the wood behind the beak was undrilled but thin enough to pass the sound, or that the hollowed space acted like a soundboard to concentrate the sound from the room onto the microphone.

The Thing was designed by Soviet Russian inventor Leon Theremin, best known for his invention of the theremin, an electronic musical instrument. In Russian, the device is called Zlatoust (Златоуст) and belongs to a class of devices called endovibrators (эндовибраторы).

The device, embedded in a carved wooden plaque of the Great Seal of the United States, was used by the Soviet government to spy on the US. 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 Ambassador Harriman, as a "gesture of friendship" to the Soviet Union's war ally. It hung in the ambassador's Moscow residential study at Spaso House for seven years, until it was exposed in 1952 during the tenure of Ambassador George F. Kennan.