ARCING OF CONTACTS IN TELEPHONE SWITCHING CIRCUITS 



1237 



measuring the voltage change across a 10-ohm non-inductive resistor 

 inserted between the main circuit condenser and ground. 



(3) Main condenser voltage measurements, Fig. 3(b): the oscilloscope 

 plates were shunted by an 1100 X 10"^^ farad capacitor and the com- 

 bination was used as the main circuit condenser. The above sets of 

 measurements 1 and 2 furnished the data necessary for a quantitative 

 estabhshment of the theory. It is evident that the measuring scope cir- 

 cuits did not interfere with the normal behavior of the contact circuit. 



Development of Circuit Current During Arc Initiation Period 



The circuit in Fig. 3(a) was used. The contact separation was gradually 

 decreased until the discharges started and the resulting current transients 

 were recorded. The circuit parameters were chosen such that, according 

 to Equation 3 of our analytical results, the initiation time was of the 

 order of microseconds. Fig. 4(A) shows a typical current-time transient 

 where a steady arc was established. The circuit had L = 1100 X 10~^ 

 henry and C — 10"^ farad. The current started from zero and increased 

 during the multiple discharge period until point 1 where the current 

 was 0.2 ampere and a sustained arc was established. The current then 

 increased to a maximum of 1.7 amperes then dropped. At point 3 the 

 arc stopped when the current was 0.24 ampere. In Fig. 4(B) the first 



(b) 

 Fig 3 _ (a) Contacts circuit and circuit current measuring circuit, (b) Con- 

 tacts circuit and main condenser voltage measuring circuit. 



