ARCING OF ELECTRICAL CONTACTS IN TELEPHONE SWITCIITNC CIRCUITS 537 



T'o Absolute initial temperature 



T^ \''oltage 



Vai Arc initiation ^•oltagc 



Vgi Glow initiation voltage 



Vg Voltage drop across the contacts wtih normal glow 



a Thermal diffusivity 



(f Work function 



oj Angular frequency (fc)~^ ' 



GENERAL 



A typical circuit consisting of a battery, a coil of an electro-magnet, 

 a cable or lead and a pair of contacts is shown in Fig. 1(a). Due to the 

 usual magnetic core of the coil, this circuit presents some unnecessary 

 complications in making interpretations of the observed contact phe- 

 nomena. Since our main objective is an understanding of the basic 

 phenomena occurring between the contacts, it appeared justifiable to 

 restrict our work to circuits and circuit elements that lend themselves 

 to simple treatment. Figure 1(b) shows the circuit used in most of this 

 work. All coils used have air cores. 



When the contacts are closed, a steady state current lo = E/R is 

 established in the circuit. At the first physical separation between the 

 contacts, the circuit current will charge the capacitance C causing a 

 ^'oltage rise at the contacts at an initial rate of lo/C. In the meantime, 

 the separation between the contacts will increase. The first breakdown 

 will occur when the voltage across the contacts first reaches or exceeds 

 the arc initiation voltage corresponding to the separation attained, the 

 atmosphere in^'ol^■ed and the contact surface condition. Fig. 2 represents 

 diagrammatically the occurrence of the first discharge, abc is the arc 

 initiation voltage versus separation line for a "normal" contact." The 



COIL CABLE 



E"^ JL _L CONTACT 



I (a, ^ ^ I 



L R I 



I ^WT^ "vVv' 1 ^WO"^ 1 



E"^ ^C CONTACT 



^ . O'' . . - . . - 



Fig. 1 — (a) Tj'pical relay circuit in i)racticc. (b) Linear circuit used in this 

 stud3\ 



