features, along with the overall simplicity of the device, made it very reliable and gave it a potentially unlimited operational life. Technical details 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 in (19.7 mm) and about 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. Discovery The existence of the bug was discovered accidentally in 1951 by a British radio operator at the British Embassy who overheard American conversations on an open Soviet Air Force radio channel as the