AKCING OF ELECTRICAL CONTACTS IN TELEPHONE SWITCHING CIRCUITS 547 



300 



UJ 



o 

 > 



o 

 < 



I- 

 z 

 o 

 o 



80 



TIME,t, IN 10~6 SECONDS 



Fig. 9 — Lowering of arc initiation voltage under dynamic conditions. Tran- 

 sient on break of Pd contacts in atmospheric air. E = 50 volts, L = 0.010 henry, 

 R = 40 ohms and C = 270 X 10~'^ farad. Velocity of contact separation = 40 

 cms/sec. 



effect. In contrast to the static line, shown as a broken Une, the break- 

 down potential shows little change with separation for a major part of 

 the transient. Towards the end, it shows a gradual increase which in this 

 particular case fails to reach the static line. Figure 10(b) is a plot of the 

 ratio (Vai)dyn/(Vai)stat versus time along the transient. 



This phenomenon is attributed to residual effects in the contact gap 

 or on the contact surfaces. In this section, are discussed the possibilities 

 of the presence of residual ions, residuad atoms and residual thermionic 

 emi.ssion. 



a. Deionization Time 



This is determined by calculating the transit lime of an ion across the 

 contact gap under the applied field corresponding to the charging of the 

 contact capacitance. For simplification, the initial motion of the ions 

 and the initial field are neglected, the voltage rise is approximated by 



V/Vai = t/tch and the field is taken as V/s. 





113 



(4) 



