1498 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



103 



10^ 



CONTACT SEPARATION IN ANGSTROMS 



Fig. 3 — Arc initiation voltages for carbon contacts. 



For contacts activated in organic vapors, constant field strength lines 

 were obtained. In Fig. 5 results are shown for palladium contacts acti- 

 vated by d-limonene. The average field strength for arc initiation is only 

 0.6 X 10* volts/cm with a spread of as much as 100 per cent of the 

 average. At separations greater than 50,000A the arc was initiated by 

 the familiar spark breakdown of air. In vacuum the constant field 

 strength line should hold for larger separations possibly until it intersects 

 Cranberg's line.^ 



Breakdowns at low fields were also observed for metals with inorganic 

 films. For instance Gleichauf^^ obtained a constant field strength line of 

 0.24 X 10* volts/cm for copper electrodes in vacuum at separations of 

 the order of millimeters. Our measurements on copper contacts at sepa- 

 rations less than 10,000A have shown breakdowns at fields as low as 0.7 

 X 10* volts/cm. It is concluded that the presence of organic or inorganic 

 films on a contact usually leads to a reduced gross field strength at which 

 the breakdown will occur. The reduction can be by as much as two orders 

 of magnitude. It is possible that this reduction was only apparent and 

 the electrons actually came from the underlying metal by extraction in 

 an intense field set up by the positive ions lying on the surface of the 

 film." 



'0 P. H. Gleichauf, Electrical Breakdown Over Insulators in High Vacuum, 

 J. Appl. Phys. 22, p. 766, 1951. 



1^ F. L. Jones, Electrical Discharges in Gases, Nature, 170, p. 601, 1952. 



