1494 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



is generally considered to be the major contributor. For illustration, a 

 palladium contact, 10~* cm^ in volume, will last for more than 10^ opera- 

 tions* only if the arc energy per operation is less than 2.5 ergs. This is 

 based on an erosion rate of 4 X 10~^^ cm^ per erg.^ Furthermore, a short 

 arc with a half ampere current, lasting for only one microsecond, will 

 dissipate as much energy as 70 ergs. Contact erosion may also take place, 

 though at much lower rates for the usual ranges of current and voltage 

 in switching circuits, due to molten bridges^ on contact break and due 

 to glow discharge.^ 



In Part I of this series, was discussed the mechanism of the initiation 

 of the short arc as determined by contact and circuit conditions. Three 

 characteristics of the arc were used in the presentation without elabora- 

 tion as to their nature: (1) the arc initiation voltage, (2) the voltage drop 

 across the arc, and (3) the arc initiation and the arc termination currents. 

 These characteristics have been the subject of a recent study to which 

 this part of the series is mainly devoted. 



In the course of this study, it was found that there should be some 

 repetition of previous work to isolate effects of certain pertinent parame- 

 ters that were not previously given due consideration. 



No attempt is made here to give a complete survey of the related 

 studies in the literature. Only a few publications are referred to as typical 

 references to the subjects discussed. 



NOTATION 



C Capacitance 



Ea Energy dissipated in the arc y 



F Gross field strength between the contacts: — 



I Current 



li Arc initiation current 



Imax Maximum current in the arc 



Im Minimum arcing current or arc termination current 



* This is the actual life requirement of some contacts in existing switching 

 circuits. 



1 L. H. Germer and F. E. Haworth, Erosion of Electrical Contacts on Make, 

 J. App. Phys. 20, p. 1085, 1949. 



2 See for example: J. J. Lander and L. H. Germer, The Bridge Erosion of 

 Electrical Contacts, J. App.^ Phys. 19, p. 910, 1948. 



3 F. E. Haworth, Electrode Reactions in Glow Discharge, J. App. Phys. 22, 

 p. 606, 1951. 



* M. M. Atalla, ''Arching of Electrical Contacts in Telephone Switching Cir- 

 cuits. Part I— Theory of the Initiation of the Short Arc," B.S.T.J., 32, pp. 1231- 

 1244, Sept., 1953. 



