In the early 1930s the British Air Ministry offered a thousand pounds to anyone who could kill a sheep at 100 yards with a radio death ray. This episode explains how that frantic quest, driven by the belief that the bomber will always get through and by rumours of German engine-stalling weapons in the Black Forest, led the Tizard Committee to Robert Watson-Watt and Arnold Wilkins. Wilkins's calculation proved the death ray impossible, but his memory of aircraft disturbing Post Office shortwave receivers, and his insight that an 82-foot bomber wingspan matched the half-wavelength of a 50-metre signal, turned detection into a weapon.

From the Daventry experiment in a delivery van in February 1935 to 360-foot steel towers along the coast, we follow the third best rule of engineering, the 1937 exercise where Hugh Dowding watched raw data paraly ...