ARCING OF CONTACTS IN TELEPHONE SWITCHING SYSTEMS 1499 



Table I. — Arcing Characteristics of Contact Materials 



(1) 



Contact Material 



(2) 



(3) 



Carbon . . . 

 Nickel .... 

 Palladium. 



Silver 



Tungsten . 



(4) 



Minimum arcing 



current for 



"clean"* 



contacts. Amps. 



0.03 



0.4 



1.1 



0.8 



0.7 



* For "normal" contacts, the minimum arcing current is less by 50 per cent 

 or more. 



The results of the above section are summarized in Fig. 6. The solid 

 lines were actually measured for palladium contacts under different 

 surface conditions. The broken lines are only speculative. For a certain 

 separation and surface condition the arc will be initiated at a voltage as 

 given by the lowest corresponding line in the figure. 



VOLTAGE DROP ACROSS A SHORT ARC 



The short arc may be defined as a discharge of electricity between 

 electrodes with a voltage drop of the order of the minimum ionizing po- 

 tential of the atoms of the electrodes.* Furthermore, due to the small 

 separation between the contacts and the local high pressure metal vapor, 

 the characteristics of the established arc are independent of a surround- 

 ing atmosphere at normal or low pressures. The short arc is characterized 

 by its constant voltage for currents above a minimum value called the 

 minimum arcing current of the contact. In contrast to the short arc, the 

 long arc between contacts at a fixed separation has a voltage drop which 

 decreases with an increase in current .^^ Most arcs occurring between con- 

 tacts on both make and break of telephone switching circuits, are short arcs. 

 In Table I, Column 3, are given our measured values of the short arc 

 voltage for a few materials. 



ARC initiation AND TERMINATION CURRENTS 



The arc termination current or minimum arcing current is defined as 

 the lowest current at which the arc can be sustained. The arc is extin- 



* The arc voltage is about 50 per cent higher except for the carbon arc which 

 has a much higher voltage; see Table I, Column 3. 



^2 See for instance : K. Gaulrapp, Untersuchung der elaktrisehen Eigenschaften 

 des Abreisbogens, Ann. Phvsik. 25, p. 705, 1936. 



