Moscow, 4 August, 1945. The European chapter of World War Two was over, and the US and the USSR were pondering their future relationship.

At the American embassy, a group of boys from the Young Pioneer Organization of the Soviet Union made a charming gesture of friendship between the two superpowers.

They presented a large, hand-carved ceremonial seal of the United States of America to Averell Harriman, the US ambassador. It was later to become known simply as The Thing.

Naturally, Harriman's office would have checked the heavy wooden ornament for electronic bugs, but with neither wires nor batteries in evidence, what harm could it do?

Harriman gave The Thing pride of place, hanging on the wall of his study - from where it betrayed his private conversations for the next seven years.

He could not have realised that the device had been built by one of the true originals of the 20th Century.

Leon Theremin was famous even then for his revolutionary eponymous electrical musical instrument, which was played without being touched.

He had been living in the US with his wife, Lavinia Williams, before returning to the Soviet Union in 1938. His wife later said he had been kidnapped. In any case, he was promptly put to work in a prison camp, where he was forced to design, among other listening devices, The Thing.

Eventually, American radio operators stumbled upon the US ambassador's conversations being broadcast over the airwaves. These broadcasts were unpredictable: scan the embassy for radio emissions, and no bug was in evidence. It took yet more time to discover the secret.

The listening device was inside The Thing - and it was ingeniously simple, little more than an antenna attached to a cavity with a silver diaphragm over it, serving as a microphone. There were no batteries or any other source of power. The Thing did not need them.
It was activated by radio waves beamed at the US embassy by the Soviets. It used the energy of the incoming signal to broadcast back. When that signal was switched off, The Thing would go silent.

Much like Theremin's unearthly musical instrument, The Thing might seem a technological curiosity. But the idea of a device that is powered by incoming radio waves, and which sends back information in response, is much more than that.

The RFID tag - short for Radio-Frequency Identification - is ubiquitous in the modern economy.

My passport has one. So does my credit card, enabling me to pay for small items simply by waving it near an RFID reader.