Early in World War II, an anti-aircraft battery might fire 20,000 to 100,000 shells to bring down a single bomber, a war of attrition the Allies could not win. This episode tells the story of the proximity fuse, a technology guarded with the same secrecy as D-Day and the atomic bomb, which put a miniature radio transmitter and receiver in the nose of a shell so it could sense a target and detonate within roughly 70 feet. It began as a British concept proposed by W.A.S. Butement in 1939, tested with sheets of tin and rockets aimed at barrage balloons, and was handed to the United States during the 1940 Tizard Mission when British industry, under the Blitz, could not solve the engineering.

American physicists including Merle Tuve at Johns Hopkins and Harry Diamond at the National Bureau of Standards had to make fragile glass vacuum tubes surv ...