ARCING OF CONTACTS IN TELEPHONE SWITCHING SYSTEMS 1 lO") 



K Constant in the relation Vai = Kidf^ 

 R Resistance 



V Voltage 



Vai Arc initiation voltage 



d Minimum separation between contacts 



h Height of a metal bridge formed during one arc 



t Time 



ta Arc duration 



V Constant voltage drop across the short arc 



ARC INITIATION VOLTAGE 



Consider the simple contact circuit in Fig. 1 comprising a pair of con- 

 tacts in series with a resistor R and a variable voltage power supply. By 

 fixing the separation between the contacts and gradually increasing the 



R R, 



AMr 



CONTACTS -L_ Q 



i 



AAA. 



VARIABLE 

 POWER SUPPLY 



i 



Fig. 1 — Contact Circuit. 



voltage, an arc is usually initiated when the voltage reaches a certain 

 value ''Vai' called the arc initiation voltage. In general Vai is a function 

 of: (1) separation between the contacts, (2) geometry of the contact 

 surfaces, (3) the surrounding atmosphere, and (4) contact material and 

 its surface. 



Our experiments were limited to contacts in atmospheric air with 

 special emphasis on separations of the order of and less than the mean 

 free path of an electron in air. For larger separations the arc initiation 

 voltage follows the well known curve of the sparking potential of air/ 

 For the smaller separations where the presence of air molecules would 

 not be expected to affect the arc initiation voltage, it was pre\iously re- 

 ported that breakdowns occurred at some constant field between 0.6 

 X 10' and 16 X lO' volts/cm.'* '• ' 



In our experiments a cantilever bar, described by Pearson,* was used. 

 The setting for zero separation was determined by a 0.1 volt source, a 



^ See for instance J. D. Cobine, Gaseous Conductors, McGraw-Hill, N. Y., 



162, 1941. 



6 G. L. Pearson, Phys. Rev. 56, p. 471, 1939. 



' L. H. Germer and J. L. Smith, J. App. Phys. 23. p. 553, 1952. 



