During the First World War, the Metropolitan Police introduced their first air raid warning system in London in 1917, which consisted of police officers detonating maroons (a type of loud firework) and the "all clear" was sounded by Boy Scout buglers.
Immediately before the Second World War, a network of sirens had been established in towns across the United Kingdom; the "alert" was given by a rising and falling tone and the "all clear" by a continuous tone.
- Ian Frederick William Beckett (2006), Home Front 1914-1918: How Britain Survived the Great War, The National Archives, ISBN 978-1-903365-81-6 p. 187
- "School Radio - Warning sounds: 'All clear' siren". www.bbc.co.uk. BBC. Retrieved 31 August 2015.
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