1238 THE BELL SYSTEM TECHNICAL JOURNAL SEPTEMBER 1953 



TIME, T, IN SECONDS X 10-6 



Fig. 4 — Circuit current transients with steady arc established. 



portion of a similar transient is shown: L = 5100 X 10"^ henry and 

 C = 10"^ farad. The current build-up during the multiple discharge 

 period is shown as an interrupted line 0-1. The steady arc was established 

 at 0.57 ampere. 



From a group of similar transients, pairs of measurements were made 

 of the initiating current and terminating current of the steady arc. The 

 results are given in Table I. It is concluded from the results that the 

 initialing current and terminating current of an arc are the same. 



The high-frequency discharges at the contact do not necessarily have 

 to be followed by a sustained arc. A sustained arc is only obtained if the 

 maximum current established during the local discharges at the contact is 

 equal to or greater than the arc initiation current. Fig. 5 (A) shows a cur- 

 rent-time transient without an initiation of the steady arc: Vo = 400 

 volte, L = 5100 X 10"' henry and C = 1100 X 10"'' farad. The maxi- 

 mum current reached was only 0.13 ampere which was not sufficient 

 for initiating a steady arc. It is of interest to notice that the oscillations 

 superimposed on the zero current line following the transient can allow 

 a calculation of the local capacitances at the contact. Such a calculation 

 gave c = 7.8 X 10""^ farad. Fig. 5 (B) shows a similar transient for the 

 same circuit with Vo = 500 volts. Following the first current build-up 

 and drop, 1-2-3, the main condenser had a residual negative voltage 

 high enough to produce a reversed current build-up and drop, 3-4-5. 



Voltage Drop in Main Condenser During Arc Initiation Period 



During the peri(Kl of rapid discharges the current build-up in the 

 circuit is accompanied by a voltage drop at the main condenser. This 

 drop may corre^ipond to one of two phenomena at the contact: (1) 

 multiple discharges leading to a steady arc, and (2) multiple discharges 

 without steady arc, followed by an open circuit. Fig. 6 and 7 are re- 



