© Science Museum/Science and Society Picture Library
This is the original magnetron, which made it possible to build radar sets small enough to fit into aircraft.
The earliest radar was ground-based, employing massive 100-metre masts in the celebrated 'Chain Home' system, which helped win the Battle of Britain. But, to counter submarines and night bombers, radar had to be miniaturised to be carried on aircraft. John Randall and Harry Boot's cavity magnetron was the first practical device for producing the ultra-short radio waves at adequate power to enable the system to work. Magnetrons later formed the heart of microwave ovens.