ACTIVATION OF ELECTRICAL CONTACTS BY ORGANIC VAPORS 777 



that when the curreiit was again small the arc was localized at a new 

 position on a fresh carbon surface so that it was again a carbon arc; 

 during most of the arc time, when the current was larger, the cathode 

 surface was maintained so free from carbon that the source of electrons 

 at the cathode was palladium metal rather than carbon. For lightly 

 carbonized surfaces, which may be just as active as judged by arc dura- 

 tion or any other test that we know of, no such enhanced arc voltage at 

 the beginning or at the end of an arc has been observed. It may well be 

 that for lightly carbonized surfaces the arc voltage is characteristic of 

 carbon for a time too short to be detected by this crude means. 



1.3 Minimum Arc Current 



The current at which an arc goes out is readily found by observing 

 on an oscilloscope the potential across closing contacts discharging a 

 capacitor through a non-inductive resistor R. At extinction, the potential 

 rises from the arc voltage v to that across the capacitor Vi . The mini- 

 mum arc current is then (Fi — v)/R. An oscilloscope trace showing such 

 a determination of minimum arc current at the arc initiation potential 

 of 400 ^•olts is reproduced as Fig. 5. (See also Reference 2, Fig. 5 and 



Fig. 4 — Oscilloscope trace rep- 

 resenting the voltage across an arc 

 at the closure of very heavily car- 

 bonized electrodes. Discharge 

 through an inductance of 10~% of 

 a capacitor of 10~^*f charged to 50 

 volts. Near the beginning and 

 near the end of the arc the source 

 of electrons at the cathode was a 

 carbon surface. 



50 r- 



_) 

 O 

 > 



TIME IN MICROSECONDS 



Fig. 5 — Voltage across clean 

 palladium contacts when a capac- 

 itor charged to 400 volts is dis- 

 charged through a resistor of 200 

 ohms. The closure arc went out at 

 the minimum arc current 0.42 

 amp. 



400 1- 



o 

 > 



5 10 



TIME IN MICROSECONDS 



