556 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



Table IV ■ — Glow-Arc Transition Data for Pd Contacts 

 IN Atmospheric Air-Cathode Diameter 0.1 Cm. 



* No glow was detected with a time resolution of 1 per cent of a half period 

 7r(LC)i/2. 



t Uninterrupted glow occupied the entire half period. 



t Obtained by dividing {Ig)max bj' the total cathode area. 



transient time, to a full transient time. By calculation, the corresjDonding 

 limiting currents and limiting current densities were obtained, columns 

 7 and 8 respectively. The ratios of the limiting current densities to the 

 normal glow current density are also given in column 9. They show that 

 at the interruption of the glow discharge the current density was 5 to 7 

 times the normal glow current density. This indicates a transition from 

 normal glow to abnormal glow before the final transition into an arc. 

 One may, therefore, conclude that if glow discharge is obtained it starts 

 as normal glow which may occupy only a small fraction of the cathode 

 area. By increasing the current the cathode glow area expands at con- 

 stant current density until it covers the entire cathode area. Further 

 current increase leads to a transition into abnormal glow with higher 

 current densities. Transition of the abnormal glow into an arc occur.s 

 when the current density reaches a limiting value. This limiting ciu'rent 

 density is extremely sensitive to surface contamination and generally 



