Aspidistra

Lewis Hurst [ FT346:65] passed on a story told by his father who, during World War II, had worked at Bletchley Park. In the story a local man had had a shock after hearing a loud voice from a bucket of coal. Well, he wasn’t Robinson Crusoe. From 1943 until the German surrender, Home Guardsmen heard voices coming from rusty barbed wire entanglements, police officers heard voices from rusty street signs, and housewives heard voices from gas stoves. Sometimes they heard music, but usually it was voices, in German, which made it more alarming (although sometimes the voices were reported as just “foreign” which seems to have been concerning enough). The clue was that rusty or corroded metal was involved and all such instances were in a particular part of England. If metal has a thin layer of oxidation and is in contact with another piece of metal it will act as a diode, and will detect and demodulate an Amplitude Modulated (AM) signal, if the signal is sufficiently strong. As the current from the diode surges back and forth in the rhythm of speech, the current will heat and cool the metal causing enough expansion to make the metal act as a loudspeaker.

Of course, it has to be a powerful transmitter and, as Mr Hurst suggests, such a secret transmitter was nearby. Codenamed “Aspidistra” (after the popular song by Gracie Fields, ‘It’s the biggest… in the world’) it had been purposebuilt by the Radio Corporation of America. It consisted of the main transmitter, a 600kw monster and a 500w unmodulated transmitter usually positioned 80km (50 miles) away, which would mislead any German direction-finding. A 50kw AM transmitter is often called a “clear channel” transmitter in the industry as it will have a range of hundreds of kilometres at night. Aspidistra would have been received clearly in Eastern Europe, but its target audience was in Germany where it would pop up on a legitimate Deutschesender frequency and provide black propaganda or instructions to local authorities designed to cause chaos and confusion, and to spread the rumour that Allied spies were everywhere. It certainly made itself felt in the popular German imagination. I remember seeing a German film of the 1950s (starring, I think, Gert Frobe) where as British bombers attacked Berlin (?) a spy, identifiable as French from his beret, neckerchief and striped shirt, opens a baguette within which is concealed a radio transmitter, and directs the bombers to their targets.

I have written about Aspidistra at more length in an earlier letter [ FT303:73]. My original source was volume II of Sefton Delmer’s autobiography, Black Boomerang (Secker & Warburg 1962). So Mr Hurst’s father was correct in the origin of the voice from the bucket, but as for his speculation that a crystal found in coal had detected the radio signal – while iron pyrites (the crystal mentioned) can act as a diode, in this case it was the oxidised metal. John Alexander Faulkner Sydney, Australia