A listening device hidden in an artwork in 1945 was undetected by US security for seven years – and it's not the only example of art having been manipulated for subterfuge.

Eighty years ago, during the final weeks of World War Two, a troop of Russian boy scouts presented the US Ambassador in Moscow with a hand-carved Great Seal of the US, at his official residence – Spaso House. The gift symbolised cooperation between Russia and the US during the war, and the US Ambassador W Averell Harriman proudly hung it in his house until 1952.

But unbeknownst to the ambassador and his security team, the seal contained a covert listening device, later dubbed "The Thing" by US technical security teams. It spied on diplomatic conversations, completely undetected for seven years. By using a seemingly innocuous artwork to infiltrate the enemy and gain strategic advantage, the Soviets had pulled off the most ingenious stunt since Odysseus's Trojan Horse. But this is a true story, even if it sounds like the stuff of spy fiction.

How did The Thing work? John Little, a 79-year-old specialist in counter-surveillance, has long been fascinated by the device, and even built his own replica of it. A documentary about Little's incredible work was released this year and, following its sell-out first live viewing in May, is due to be screened on 27 September at the National Museum of Computing, at Bletchley Park, Buckinghamshire.

He describes the technology of The Thing in musical terms – as being composed of tubes like organ pipes and a membrane "like the skin of a drum, that will vibrate to the human voice". But it was compacted into a tiny object that looks like a hat pin – and with the advantage of passing unnoticed by counter-surveillance screening because it had "no electronics, no battery, and it doesn't get warm".

The engineering of such an instrument was also painstakingly precise – "a cross between a Swiss watch and a micrometre". Historian H Keith Melton has claimed that, in its day, The Thing "elevated the science of audio monitoring to a level previously thought to be impossible."

Within Spaso House, The Thing was only activated when a remote transceiver, based in a nearby building, was switched on. This sent out a high frequency signal which reflected back all the vibrations coming from the bug's antenna. It was only when a British military radio operator working in Moscow in 1951 accidentally tuned into the exact wavelength used by The Thing, and heard conversations from a far-distant room, that it was detected. The next year, US technicians swept the ambassadorial residence and – after no fewer than three days' search – realised that the hand-carved Great Seal was an invisible ear, eavesdropping on........

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