544 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



Table I — Initiation of Reversed Arcs by Overcharging of 

 Contact Capacitance 



(Calculated) 



Vai = 300 volts, i; = 14 volts. 



predicted alcove. These additional arcs have appeared to be initiated at 

 lower voltages than the first arc. This is undoubtedly due to the residual 

 surface and gap effects of the previous arc* These are discussed in the 

 following section. 



Arc Initiation Under Dynamic Conditions — Introduction 



In Reference 2 measurements have been presented of the arc initiation 

 voltage between contacts at different separations and surface conditions. 

 These tests are "static" in the sense of allowing enough time to elapse 

 between two arcs to obtain a complete reconditioning of the contact 

 surfaces and gap. With successive arcing, as obtained on break of an 

 inductive circuit or during one breakdown, it was observed that the 

 arc may be initiated at appreciably lower voltages compared with 

 static test results. 



One arc may enhance the initiation of a shortly following arc possibly 

 through the effects of: residual ions in the gap or on a cathode surface 

 film, residual metal atoms in the gap and residual thermionic emission. 

 Exactly how each of these effects can enhance the initiation of the arc 

 can be determined only after an understanding of the mechanisms of 

 initiation of the first arc, its maintenance and its termination. It is in 

 order at this point to present a sketchy outline of some plausible mecha- 

 nisms which are largely of speculative nature. This discussion is also 

 limited to short arcs initiated and maintained with no direct influence 

 of the surrounding atmosphere. 



* The additional arcs observed may be partially accounted for by a considera- 

 tion of the actual value of the arc terminating current which was taken as zero in 

 the above calculations. 



