ARCING OF CONTACTS IN TELEPHONE SWITCHING SYSTEMS 1505 



Table II — Effect of Polarity on the Characteristics of the 

 Arc — Palladium-Carbon Contacts 



(1) 



Contact Configuration 



C+, Pd- 



Pd+, Pd 



C-, Pd+ 



C-, c+. . 



(2) 



Arc initiation, 



voltage at 6000A 



separation 



300 

 320 

 130 

 120 



(3) 



Arc voltage 



13-15 

 14-15 

 20-30 

 20-43 



Minimum arcing 

 current. Im amps. 



0.2-0.5 



1.1 

 0.2-0.3 



0.03 



loose particles. The separation between the contacts was set at 6000A. 

 By gradually raising the voltage across the contacts and observing it on 

 a cathode ray oscilloscope, the arc initiation voltage was determined. 

 The same measurement was repeated several times. Preceding each mea- 

 surement, the contacts were recleaned by the same process explained 

 above. The polarity was then reversed and a new set of measurements 

 was taken. The results are shown in Table II, Column 2. They indicate 

 that the arc initiation voltage is determined by the cathode. This 

 furnishes support to the postulate that field emission is the first step of 

 the mechanism of arc initiation. By recording the voltage across the 

 contacts during the arc, measurements were made of the arc voltage and 

 the minimum arcing current. The results are given in Table II, Columns 

 3 and 4. The arc voltage measurements indicate rather conclusively that 

 the arc voltage is determined by the cathode. The minimum arcing cur- 

 rent measurements, however, were only slightly, yet consistently, higher 

 with a palladium cathode. It is thought that during a single short arc, 

 particularly with the high intensity arcs used, there is a certain amount 

 of exchange of materials between the electrodes. This exchange is possibly 

 responsible for the observed influence of the anode on the arc character- 

 istics. 



It is concluded that the arc initiation voltage as well as the arc voltage 

 are characteristics of the cathode while the minimum arcing current 

 seems to be influenced by both electrodes with stronger inclination 

 towards the cathode characteristics.* The following reservation, how- 



* Early experiments by H. E. Ives^' have led to the conclusion that the arc 

 voltage is a characteristic of the anode. In his experiments, however, currents 

 of the order of one ampere were established in the circuit while the contacts were 

 closed. Arc measurements were made during the subsequent break of the contacts. 

 It is thought, therefore, that metal bridges must have formed during the break, 

 transferring metal from the anode to the cathode. ^ The arc produced must have 

 been influenced accordingly. In our experiments this difficulty was entirely 

 eliminated. 



13 H. E. Ives, Minimal Length Arc Characteristics, J. Franklin Inst. 198, No. 

 4, 1924. 



