Air raid warning system by telephone. The Home Office, in conjunction with the General Post Office, have now perfected a system of air raid warnings by telephone, in which a card bearing the telephone numbers of air raid wardens, ambulances etcetra are handed to telephone operators in three stages. These are; 1) on the approach of enemy aircraft; 2) enemy planes overhead; and 3) all clear. An operator receiving the first card for warning of the approach of enemy aircraft'. Photograph by Harold Tomlin.
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