[Elsie]

Which cavity?

The entire oven space or the one in the tube?

[RadioAstronomer]

http://www.answers.com/topic/magnetron

http://www.sciencemuseum.org.uk/on-line/electron/section5/magnetron.asp

http://www.absoluteastronomy.com/encyclopedia/M/Ma/Magnetron.htm

http://www.tpub.com/neets/book11/45i.htm

:-)