English: Microwave cavities in original cavity magnetron tube, developed by John Randall (1905-1984) and Harry Boot (1917-1983) at Birmingham University in 1940. This was contained inside a vacuum tube. A heated cathode extended up the central axis, which emitted electrons, and a permanent magnet embracing the tube created a magnetic field along the axis of the tube. The magnetic field caused the electrons to circle in bunches within the central cavity of the tube, inducing microwave oscillations in the 6 peripheral cylindrical cavities.
The cavity magnetron was the first practical device for producing power at centimeter wavelengths. It enabled the development of radars that could produce clear images of distant objects by bouncing microwaves off their targets. These played a significant role in World War 2. Please note: this image has not been fully finished.