COLD CATHODE GAS TUBES FOK TELEPHONE SWITCHING SYSTEMS 7(31 

 220 



210 



200 



In 190 



o 

 > 



J 180 



O 

 Q 



5 170 

 a. 



160 



150 



MO 



1 23456789 



SPACING, d, IN CM X PRESSURE, po, IN MM OF Hg 



10 



Fig. 3 — Breakdown voltage as a function of spacing and pressure for parallel 

 plane anode and cathode. 



anode and cathode geometry, a molybdenum cathode, and neon filling 

 gas. The curve is plotted as a function of the product of pressure po 

 in mm Hg and electrode separation d in cm. Approximately the same 

 plot would obtain for other pressures because both 77 and 7 are functions 

 of (E/po) and, for uniform fields, E is simply the voltage divided by the 

 separation 



(E) ^1^1. 



(at breakdown Cl Po 



(Po). 



(12) 



Since the variation of 7 with E/po is small and may be ignored in this 

 elementary discussion, the minimum breakdown ^'Oltage corresponds 

 ver}^ nearly to the optimum value of the ionization coefficient rj. At 

 spacings or pressures less than optimum, t/ is reduced because some elec- 

 trons strike the anode without colliding with gas atoms. At spacings or 

 pressures greater than optimum, rj is reduced because electrons do not 

 gain enough energy between collisions to ionize efficiently. 



It can be seen that a way of meeting the switching reciuirement of 

 constant breakdown \'oltage would be to design the tube to operate at 

 the minimum of Fig. 8. Minor changes in spacing or filling pressure from 

 one tube to another and changes in pressure with tube operation would 

 result in small changes in breakdown voltage. The advantages of op- 



