In February 1940, in a cramped lab at the University of Birmingham, John Randall and his graduate student Harry Boot built a device the size of a hockey puck that would, according to American historian James Phinney Baxter III, become "the most valuable cargo ever brought to our shores." It was the resonant-cavity magnetron, and it changed the war.

The problem was simple to state and maddening to solve. Radar in 1939 used long wavelengths, which meant enormous antennas and fuzzy resolution — good enough to spot a bomber squadron over the English Channel, useless for finding a U-boat conning tower in a rolling sea. Everyone knew the answer was microwaves: shorter wavelengths, smaller antennas, sharper pictures. Nobody could generate microwaves at any useful power. Existing magnetrons produced milliwatts.

Randall and Boot's insight was almost embarrassingly elegant. They took the standard magnetron — an anode block with a cathode down the middle, immersed in a magnetic field — and drilled a ring of resonant cavities around the anode, like holes bored around the edge of a wagon wheel. Each cavity acted as a tiny tuned circuit. Electrons spiraling past the openings excited the cavities into oscillation, and the cavities in turn bunched the electrons into a rotating "spoke" pattern that pumped energy back into the resonators. Positive feedback, at 3 GHz, at kilowatt power levels. Their first prototype, cobbled together with sealing wax and a repurposed electromagnet from an old spectroscopy rig, produced 500 watts of microwave power — a thousandfold leap over anything before it.

The rest is a story of desperate wartime technology transfer. In September 1940, the Tizard Mission carried a working magnetron across the Atlantic in a black metal deed box and handed it to a stunned American physics establishment. MIT's Rad Lab was founded around it. By 1943, H2S ground-mapping radar was letting RAF bombers see cities through cloud. By 1944, ASV Mk III was hunting U-boats so effectively that Dönitz withdrew them from the North Atlantic. Percy Spencer at Raytheon, walking past an active magnetron in 1945, noticed a chocolate bar melting in his pocket — and the microwave oven was born.

Randall himself is the strangest part of the story. After the war he pivoted completely: he took a chair at King's College London and built the biophysics unit where Rosalind Franklin and Maurice Wilkins would produce the X-ray diffraction images of DNA. The man who lit up radar screens across the Allied fleet went on to run the lab that produced Photograph 51. Two of the twentieth century's defining images — a bomber's-eye view of Hamburg, and the double helix — trace back through the same office door.