1240 THE BELL SYSTEM TECHNICAL JOURNAL SEPTEMBER 1953 



for the initiation of the arc, a small local capacitance c' will furnish the 

 necessary charge, through a small impedance z' ^ according to whatever 

 mechanism that may be involved in the initiation process.* The con- 

 nection from the contact to the main circuit is represented by a short 

 transmission line \vith the distributed characteristics, r, i and c. The 

 drop at the contact from the initial voltage Fo to the arc voltage v will 

 cause a current surge (To — "o) {c/tf^, if r is neglected, for a period 

 2{(cf'^ corresponding to the time required by the pulse to travel to the 

 end of the line and return to the contact. At this time the arc is ex- 

 tinguished by the reflected pulse and the final voltage at the contact 

 is — (Fo — 2v). Figs. 8(b) and 8(c) are diagramtic representations of 

 the process. For a purely inductive line, therefore, the contact voltage 



V following one discharge is — (Fo — 2v). For a dissipative line, however, 



V is algebraically greater. Equation 4 of Germer and Haworth^ was 

 derived to give the voltage following an arc for a similar circuit with 

 lumped characteristics. For r/{t/cf''^ less than 1.0 this equation can be 



220 



220 



10 950 



TIME, T, IN SECONDS XIO'^ 



Fig. 6 — Voltage transients across main condenser with steady arc established. 



220 



70 



TIME,T, IN SECONDS X 10"^ 



JPig^7 — Voltage transients across main condenser without steady arc. 



This mechanism is not clearly understood at the present time. It is the opinion 



of the writer, however, that the initiation time is only a few times the transit time 



\r ^T^^ *°" ^^^^^ *^® prevailing conditions. 



r'r^r^^^^ *"^ ^- ^ Haworth, Erosion of 



Appl. Phys. 20. p. 1085, 1948. 



Electrical Contacts on Make, 



