Early microwave magnetrons

The first description of the static characteristics of a device called magnetron was described by A. W. Hull in 1921. The tube had a linear filament in a coaxial cylindrical plate. The plate cylinder was surrounded by a winding, used to generate a magnetic field. The radial electric field and the superimposed magnetic field forced electrons to follow circular orbits before reaching anode. If the magnetic field was raised, the radius of the orbits became smaller and smaller until electrons could not reach the plate, so causing a cut-off condition in the anode current. Hull also noted that just on the border between conduction and cut-off, adding a resonant tank circuit, the tube could sustain selfoscillations.