Bugs in espionage

'In 1946, Soviet school children presented a two-feet wooden replica of the Great Seal of the United States to the American ambassador. He hung the seal at his residence. A bald wooden eagle, which was part of the seal, had been bugged. In 1952, an inspection revealed that the eagle contained a microphone and a passive resonant cavity that could be stimulated from an outside radio signal. The equipment had no power source or transmitter making it much harder to detect. The cavity had a metallic diaphragm that moved in unison with sound waves from a conversation in the room.'
More:
http://m.economictimes.com/news/news-by-industry/et-cetera/biggest-espionage-dramas-where-bugging-devices-played-a-key-role/articleshow/8948405.cms

Espionage terms explained: http://www.spywriter.com/terms.html