1496 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1953 



10,000 ohms resistor and a cathode relay oscilloscope. The zero setting 

 could be repeated with a precision of =b 500A. All the reported results 

 were obtained by fixing the contact separation, raising the voltage and 

 observing the breakdown on a cathode ray oscilloscope. Contacts tested 

 were given one of three different surface treatments: 



(1) The contact surface was polished with fine emery paper, washed 

 with methyl alcohol, then exposed to the laboratory atmosphere for a 

 few hours. After about 5 arc discharges, readings of arc initiation voltage 

 seemed to vary at random. Contacts thus treated are referred to as 

 ''normal" contacts. 



(2) Contacts were subjected to arcing for about 5 minutes at the rate 

 of 15 arcs per second. Arcing was produced by the discharge of a half 

 microfarad condenser at 500 volts through a 10-ohm resistor. Measure- 

 ments of the arc initiation voltage followed immediately after this treat- 

 ment. These contacts are referred to as "clean" contacts. Their behaviour 

 usually changed to that of ''normal" contacts after a short exposure to 

 the laboratory atmosphere. 



(3) Contacts were subjected to arcing at the rate of 3 arcs per second 

 for about one hour in air saturated with d-limonene. The arc was pro- 

 duced by discharging a 0.1 -microfarad condenser at 50 volts through a 

 100-ohm resistor. These contacts are referred to as "activated" con- 

 tacts.* 



Fig. 2 shows the results obtained with "normal" palladium contacts. 

 Each point represents the average of five readings. The maximum spread 

 was 40 per cent of the average. For separations less than 10,000 A, about 

 two mean free paths of an electron in air at normal conditions, a constant 

 gross field strength line of 3 X 10^ volts/cm was obtained. At larger 

 separations the measured arc initiation voltages were essentially the well 

 known sparking potentials of air. Fig. 3 shows the corresponding results 

 obtained with "normal" carbon contacts in air. Below a separation of 

 15,000 A, the arc was initiated at a constant gross field strength of 2.4 

 X 10 volts/cm. The maximum spread of the individual points was only 

 15 per cent of the average. 



In the absence of air, it is expected that the constant field strength 

 lines will hold for higher separations.* In Table I, Column 2 are given 

 the measured values of the gross field strengths at which the arc was 

 initiated for a group of "normal" contact materials. 



* Recent unpuhlinhod measurements by Dr. P. Kisliuk on similar contacts in 

 vacuum have indicated that the constant field strength relation Wi = F((i) holds 

 initially for larger separations and is followed by a gradual transition into a 

 8(juare root relation Vai — K(d)^l^ as proposed by Cranberg.^ 



» L. H. Germer, Arching at Electrical Contacts on Closure —Part I, J. Appl. 

 Phys. 22. p. 955, 1951. 



