1384 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



which has been carefully evacuated and filled to a fraction of an atmos- 

 phere with a rare gas such as neon. 



A typical static volt-ampere characteristic of the structure of Fig. 10 

 is shown in Fig. 11. This curve was taken with a cathode having a hollow 

 portion 3^" long and 3^6" deep with a cathode gap of 0.023''. A neon 

 filling pressure of 58 mm of mercury was used . It can be seen that there 

 is the usual low-current negative slope associated with the transition 

 from breakdown (current range III of Fig. 1). A new characteristic of 

 interest is the second region of negative slope in the abnormal glow range 

 of currents. It has been found that this second region can be made stable 

 with time in a given tube and reproducible from tube to tube. It is also 



tzA 



DEPTH OF HOLLOW 



/ 

 / 



/ / 7 



CATHODE GAP - 





CATHODE 



ANODE-CATHODE 

 SPACING 



LENGTH OF 

 HOLLOW 



ANODE 



Fig. 10 — Electrode geometry of a hollow cathode tube. 



found that the tube impedance has a negative resistance component over 

 the voice frequency range. Thus, this second region of negative resistance 

 offers attractive possibilities as a transmission element. 



The impedance of this tube at 300 and 3,000 cps is shown in Fig. 12 

 for the same range of operating current as Fig. 11. The optimum current 

 for negative resistance and the value of negative resistance are functions 

 of the cathode gap, but so long as the other cathode dimensions are 

 constant the optimum current is relatively independent of the density 

 of the filUng gas. 



A useful way of studying the interrelation of cathode gap and filling 

 pressure is shown by Figs. 13 and 14. For these data the length and 

 depth of the hollow portion were kept constant at the values of ]/^" and 

 \{^" respectively. Fig. 13 shows the resistive component of impedance 

 as a function of frequency for different filling pressures ^vith a fixed 

 cathode gap. 



