1380 THE BELL SYSTEM TECHNICAL JOUENAL, NOVEMBER 1953 



800 1200 1600 2000 

 FREQUENCY IN CYCLES PER SECOND 



2400 



2800 



Fig. 6 — Resistive and inductive components of 313-C starter gap impedance 

 as a function of frequency. 7dc = 25 mm. 



inductive reactance is related to the current approximately as follows : 



wL = 5 + f- . 



(1) 



B and C are functions of frequency. 



If we wish to use tubes of this type in central office switching circuits 

 which pass voice frequency currents, we find that even one tube con- 

 nected between a 600-ohm source and a 600-ohm load gives a prohibi- 

 tively high insertion loss. The impedance level of the voice frequency 

 circuit may, of course, be raised up to 4,000 or 5,000 ohms. But practical 

 switching circuits might require as many as four tubes in series. Hence, 

 we see that the impedance of tubes of this type severely limits their 

 usefulness. 



From Fig. 7 we might argue that to get low resistive components we 

 might continue to increase the direct current flowing through the tube. 

 However, in this range of currents, the life of the tube varies approxi- 

 mately as \/l\c while the resistive component varies as l/Idi^. 



Over the range of currents plotted in Fig. 6, the cathode is fully covered 

 with glow. As noted above, the resistive component is 





(2) 



