Jan Olieslagers: .....One of the last great British inventions, I should think.......
Wikipedia: The first simple, two-pole magnetron was developed in 1920 by Albert Hull at General Electric's Research Laboratories (Schenectady, New York), as an outgrowth of his work on the magnetic control of vacuum tubes in an attempt to work around the patents held by Lee De Forest (also US) on electrostatic control.