Original Cavity Magnetron, 1940.
© Science Museum / Science & Society Picture Library
This was developed by John Randall (1905-1984) and Harry Boot (1917-1983) at Birmingham University. Radars that could produce clear images of distant objects played a decisive part in WW2, and worked by bouncing powerful, ultra-short radio waves off their targets. The cavity magnetron was the first practical device for producing such waves.