The air raid warning system was developed in 1938/9 and would become very familiar to those living in large cities.
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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