ARCING OF COXTACTS IX TELEPHOXE SWITCHING SYSTEMS 1497 



2000 



1000 

 800 



600 



0) 400 

 < 



g 200 



z 



^ 100 



^ 80 



60 



40 



20 



Fig. 2 



2 ^ « « ,04 2 ^ « « 10^ 2 



CONTACT SEPARATION IN ANGSTROMS 



Arc initiation voltages for "Normal" palladium contacts. 



Due to the spread of the above measurements and their observed de- 

 pendence on surface exposure and treatment it was suspected that the 

 constant field strength characteristic obtained was due to surface con- 

 tamination. This was verified by testing contacts cleaned by the heavy 

 arcing process explained above. The results are shown in Fig. 4. The 

 familiar constant field strength line was not obtained for separations as 

 low as 1500 A. Instead, the arc was initiated at voltages comparable to 

 the sparking potentials of air. Since the separations were too small, the 

 smallest being three times less than the mean free path of an electron in 

 air, it was thought that the effect was due to some mechanism invohdng 

 the adsorbed air molecules or due to breakdown along a longer path at 

 the Paschen's minimum potential.^ In the absence of air, it is, therefore, 

 expected that higher voltages and higher field strengths in excess of 20 

 X 10^ volts/cm, as obtained at 1500A, will be needed to initiate the arc. 

 It is possible that Cranberg's relationk^ V = K{df''^, will hold for separa- 

 tions as low as a few thousand angstroms. In Fig. 4, this relation, with 

 K = 10' volt, cm"''', is plotted. 



9 L. Cranberg, The Initiation of Electrical Breakdowns in Vacuum, J. Appl. 

 Phys. 23, p. 518, 1952. 



