COLD CATHODE TUBES FOR AUDIO FREQUENCY SIGNALS 1375 



less paraUel to each other and closely spaced. The optimum dimensions 

 are a function of the density and kind of filling gas used. Electrons and 

 ions generated in the cathode fall and negative glow regions of one 

 cathode can be more efficiently used in producing new electrons and ions 

 if they can enter another cathode fall region instead of diffusing outward 

 into the Faraday dark space. This effect is particularly noticeable in the 

 abnormal glow range of currents. 



This is illustrated by the curves of Fig. 2 which were taken on a tube 

 containing two parallel plane cathodes with an adjustable spacing be- 

 tween them. Because Fig. 2 is plotted to a linear current scale, current 

 ranges I and II of Fig. 1 are compressed to the left-hand axis. The upper 

 curve is obtained when the cathodes are far apart so that there is no in- 

 teraction. The lower curve is obtained when the spacing is close enough 

 to give a hollow cathode effect. 



The sustaining voltage of a normal glow discharge is dependent upon 

 the anode to cathode distance. To investigate this we can arrange a 

 parallel plane cathode-anode structure with the anode attached to a piece 



16 



16 



20 



2 4 6 8 10 12 14 



CURRENT IN MILLIAh/PERES 



Fig. 2 — Volt-ampere characteristic of parallel plane cathodes at two different 



spacing. 



