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BELL SYSTEM TECHNICAL JOURNAL 



existence of sufficient resistance in the circuit connected to the second 

 electrode, it will go through the model coming to rest on the celluloid 

 surface above. With no resistance in the external circuit this curious 

 behavior will occur at the vertical part of the cliff indicated by the 

 bounding curve c. The presence of some resistance will cause this to 

 occur for larger values of Zy. The exact place can of course be 

 determined by noting the point of tangency between a resistance line 

 drawn on Fig. 9 and one of the characteristic curves — just as the 

 stable operating conditions for any negative resistance device is found. 



Fig. 13 — A different view of the concave surface of the model shown in Fig. 12. 

 (The celluloid surface has been removed.) 



The entire surface is stable if the external resistance is greater than 

 approximately 0.25 in units of (pjZ'i. 



Illustrative Examples 



I. Class B Operation of a " Critical Distance'' Tetrode 



Since the purpose of "critical distance" operation is to prevent the 

 passage of secondary electrons from the plate (our second plane) to 

 the screen grid (our first plane) , the existence of a potential minimum 



