ARCING OF KLECTHICAL CONTACTS IN TELEPHONE SWITCHING CIRCUITS 553 



Table II — Cathode Current Density in Steady Normal Glow 

 IN Dry Atmospheric Air at 25°C 



* Measurement b}- F. E. Haworth.'^ 



here of the process of the initiation of the steady arc which was explained 

 in detail in reference.'' For the inductive circuit in Fig. 13, when the 

 proper contact separation is reached, a first breakdown will occur dis- 

 charging the local capacitance at the contacts. This is followed by re- 

 charging from C through L and a second breakdown. This will repeat 

 w hile the circuit current will increase in a discontinuous fashion. If it 

 leaches the minimum arcing current of the contact, a steady arc is es- 

 tablished, otherwise, the transient will be made up entirely of local 

 multiple discharges. Figures 14D and B are the main condenser voltage 

 transients corresponding to the above two cases respectively. 



The interrupted arcs, or multiple discharges, and the steady arc con- 

 stitute the two processes of conduction that are commonly obtained when 

 the ^•oltages involved are below the spark breakdown potential of the 

 surrounding atmosphere. In such cases, the arc initiation is dependent 

 on the contact material and its surface condition and is independent of 

 the atmosphere." If the voltages involved are equal to or greater than 

 the minimum sparking potential of the atmosphere, the initiation of a 

 l)ieakdown is primarily dependent on the atmosphere. This breakdown, 

 llowe^'er, may in addition lead to a glow discharge as discussed above. 

 This immediatel}^ raises the question as to whether breakdowns leading 

 to an arc and breakdowns leading to a glow discharge are initiated at the 

 same potentials. For this purpose the following experiment was per- 

 formed. 



POWER SUPPLY 



i 



^OSCILLOSCOPE 



1 



Fig. 13 — Simplified circuit for the study of glow-arc transitions. 



