In the American Embassy in the Soviet Union, installed in the Great Seal of the United States, was a secret, analogue bug that transmitted sound from inside the embassy without any electronics or batteries. Oh, and the it was invented by the same guy that made the Theremin instrument, Leon Theremin. Amazing.
The Thing, designed by Léon Theremin,[2] was very simple by today’s standards. It 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. Sound waves caused the membrane to vibrate, which varied the capacitance “seen” by the antenna, which in turn modulated the radio waves that struck and were retransmitted 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.
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