From: Chuck Guzis
the magnetron was made out to be a super-secret
device, yet there's a
clear explanation of it in my 1942 "Radio Handbook".
Ordinary magnetrons had indeed been around for a while; they were invented in
1920. The British invention was the _cavity magnetron_, a quite different
beast; it was kind of a cross between a magnetron and a klystron, with the
best features of each.
Buderi (which is indeed an excellent history, perhaps the best in the radar
section of my library) has a good explanation of how it works.
Noel