The Thing, also known as the Great Seal bug, was one of the first covert listening device

Covert listening device

A covert listening device, more commonly known as a bug or a wire, is usually a combination of a miniature radio transmitter with a microphone. The use of bugs, called bugging, is a common technique in surveillance, espionage and in police investigations.A bug does not have to be a device...

s (or "bugs") to use passive techniques to transmit an audio signal. Because it was passive, being energized and activated by electromagnetic energy from an outside source, it is considered a predecessor of current RFID technology.
The Thing, designed by Léon Theremin

Léon Theremin

Léon Theremin was a Russian and Soviet inventor. He is most famous for his invention of the theremin, one of the first electronic musical instruments. He is also the inventor of interlace, a technique of improving the picture quality of a video signal, widely used in video and television technology...

, was very simple by today's standards. It consisted of a tiny capacitive membrane connected to a small quarter-wavelength antenna

Antenna (radio)

An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

; it had no power supply or active electronic components.