ACTIVATION OF ELECTRICAL CONTACTS BY ORGANIC VAPORS 783 



trodes, 2.4 X 10*^ volts/cm, and our unpublished measurements agree 

 with this. The striking field for carbon surfaces is thus only a little less 

 than that found for cathode arcs at clean metal surfaces (Reference 4, 

 Fig. 8), and very different from the field at which arcs strike between 

 active surfaces.* 



In another experiment, tests were carried out upon carbon particles 

 in the 4 to 5 X lO"** cm range of diameters, deposited sparsely upon a 

 palladium surface as before. A careful comparison was made of the elec- 

 trode separations at which an arc struck at 50 volts and at 250 volts. At 

 the higher ^'oltag■e, the distance was greater than at the lower voltage 

 by the factor of only l.o, offering confirmation that the isolated carbon 

 particles act chiefly as chunks of material, partially closing the electrode 

 gap. 



The one Avay in which the carbon that produces activation differs from 

 other carbon, and in particular from small ca^^bon particles dusted 

 sparsely upon a smooth metal surface, is in the very large number of 

 its particles and in its state of subdivision. This gives an eminently 

 plausible clue to the great electrode separation at which breakdown 

 occurs between active surfaces. According to this model, breakdown 

 occurs at a great separation between active surfaces because, at the 

 electric field corresponding to this separation, electrostatic forces become 

 sufficient to cause motion of small particles which decreases the separa- 



* In measuring the striking field at carbon surfaces for low voltages by the 

 oscilloscopic method, a value of the order of 0.6 X 10" volts/cm was found earlier 

 (Reference 6, Table I). This result was certainly in error, because of burning of 

 carbon in the arc, so that the separation of the electrodes when the arc ended was 

 greater than it was at the arc initiation. 



To check this explanation of the earlier incorrect result, an experiment was 

 carried out in which the time to closure for carbon electrodes was measured as a 

 function of the energy in the arc. In successive tests, a number of different capaci- 

 tors, each charged to 50 volts, were discharged on the closure of carbon electrodes. 

 The time to closure was foiuid to increase progressively with capacitance for the 

 values 10^, 10', 10' and 10^ /ifii. Carrying out the measurements man\' times and 

 taking average values, it was found that the time to closure increased linearly 

 with the cube root of the capacitance. This suggests strongly that a hole was 

 being burned in one of the electrodes and the increased time to closure was just 

 the time for one electrode to move the depth of the hole. A quantitative value 

 for the volume of the hole can be obtained from the data, on the basis of an as- 

 sumed hole shape. In earlier work (Reference 9, page 1088), a pit on a metal elec- 

 trode was assumed to be a spherical segment with the depth equal to one-half 

 of the pit radius. Making the same assumption for the hypothetical hole in the 

 present tests, and assinuing an electrode velocity on closure of 30 cm/sec, it turns 

 out that the relationship between volume of the hole and energy of the arc is 

 V = 4.5 X 10^'2 emVefg- The agreement of this resvdt with that for the erosion 

 of the metal anode in an anode arc (Reference 9, page 1088), is remarkable and 

 must be largely fortuitous. The agreement does, nevertheless, make almost cer- 

 tain that burning of one of the electrodes (the cathode, as we know from other 

 work) is the reason for the oscilloscopic method giving incorrect values for the 

 electrode separation at which an arc strikes l)et\veen carbon elect roiies (Refer- 

 ence 6, Tatile I). 



