The 1940 breakthrough
John Randall and Harry Boot at Birmingham University drilled six resonant cavities into a copper block, added a heated cathode in the middle, and applied a magnetic field. The result: 10 kW of pulsed 10 cm microwaves from a device you could hold in your hand. Existing klystrons produced milliwatts.
The Tizard Mission
In September 1940 Britain sent a working magnetron to the United States in a black metal trunk. Historian James Phinney Baxter called it 'the most valuable cargo ever brought to our shores.' MIT's Radiation Lab reverse-engineered it into the H2S airborne radar, the SCR-584 fire-control radar, and dozens of other systems.
Why microwaves mattered
Long-wave radars needed enormous antennas. A 10 cm microwave radar could use a 1 m dish — small enough for aircraft, ships and trucks. Suddenly, fighters could find U-boats at night, bombers could see cities through cloud, and fleets could spot incoming kamikazes.
Same physics in your microwave oven
Every kitchen microwave still uses a cavity magnetron, almost unchanged from Randall and Boot's design. The same 2.45 GHz that boils your soup once guided a Lancaster bomber to its target.