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BELL SYSTEM TECHNICAL JOURNAL 



Fig. 25 — An experimental model of the split-plate magnetron showing a possible 

 arrangement of the magnetic field. 



The magnetron depends for its operation upon the curvature of the 

 electron orbits produced by the magnetic field. As first shown by 

 Hull ^^ in 1921, a critical field exists beyond which the anode current 

 falls off rapidly to zero. This field is given by the relationship 



// 



6.72 

 R 



Vf, 



(3) 



where R is the anode radius and V is the potential of the cylindrical 

 anode with respect to an axial filament. Although the original 

 magnetron of Hull and Elder made use of variations in the magnetic 

 field in its operation as a generator, it was soon discovered that 

 oscillators could also be produced with steady fields by two somewhat 

 different mechanisms. The one, first pointed out by Habann,^^ makes 

 use of a negative resistance effect observable in the static charac- 

 teristics and the other, first described by Zdcek,*^ involves the electron 

 transit time in a way quite analogous to the way in which it is involved 

 in the positive grid triode. Both mechanisms have been used to 

 produce oscillations at ultra-high frequencies. 



