The air raid warning system was developed in 1938/9. The sound of the 'warning' signal and the 'all clear' would become very familiar to people living in large cities in Britain.
Air raid sirens were placed on the top of tall buildings - often police stations or similar - or on the top of poles if no suitable building could be found.
In cities the sirens were powered electrically and produced two signals.
The first was the warning - a rising and falling signal, created by varying the power to the siren.
The second was the all clear - a single, continuous note. After it was confirmed the skies were clear of enemy aircraft the 'all clear' would sound. It brought relief to countless thousands as they made their way wearily out of shelters to resume daily life.
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Commemorate 100 years since WW1 by staging our specially-written play 'Archie Dobson's War'.
All 13 episodes of Michael Morpurgo's moving WW1 story are available to listen to online.
Notes to support the programmes including details of all the series content.