COLD CATHODE GAS TUBES FOR TELEPHONE SAVITCHING SYSTEMS 759 



10 



10 



'2 10-'° 10-« 10"^ 10"^ 

 TUBE CURRENT IN AMPERES 



10 



Fig. 2 — Voltage versus current curve of typical gas diodes. 



of the gas or very small photoelectric emission of electrons from the 

 cathode are commonly used for this auxiliary supply. A typical voltage- 

 current curve is shown in Fig. 2. At low voltage, the current is very 

 small, often being in the range of lO^^"* ampere or less. The current in- 

 creases with the voltage because collisions of electrons with neutral gas 

 atoms produce additional excitation and ionization in the gas. Some of 

 the new ions and excited gas atoms release new electrons by secondary 

 emission when they strike the cathode. 



The rate of increase of current with voltage depends on the kind and 

 the pressure of the gas filling, the cathode material, and the tube geome- 

 try. An important characteristic of the gas is defined by an ionization 

 coethcient rj, which represents the number of new electrons (and ions) 

 produced by a single electron moving through the gas a distance corre- 

 sponding to one volt of potential difference.^ This coefficient is a function 

 of the kind of gas and of the ciuantity E/po where E is the voltage gradi- 

 ent and po is the normalized gas pressure. The fact that there is an opti- 

 mum E/po at which tj is a maximum will be important to later discussion. 

 The electron current at the anode, 4 , produced by gas amplification of a 

 photoelectric current I'n at the cathode is^ 



?u 



U>c 



Jvo '"'^■ 



(7) 



