ACTIVATION OF ELECTRICAL CONTACTS BY ORGANIC VAPORS 775 



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NUMBER OF OPERATIONS IN LIMONENE VAPOR 



1600 



Fig. 2 — Breakdown distance, and apparent striking field, for arcing at relay 

 contacts on closure in the presence of limonene vapor, plotted against number 

 of operations. Each closure discharges a very small capacitor charged to 50 volts. 

 Contacts are clean and inactive at the beginning of the test. 



resents the average of 100 separate measurements. During tests of this 

 sort, it was discovered that, with frequent microscopic examination of 

 the electrodes, black sooty material could easily be seen after the first 

 30 closures, before certain evidence of activation could be obtained in 

 any other known way. In Section 4.1, it is shown that this material is 

 carbon. 



The average electrode separation at which an arc struck between clean 

 electrodes at the beginning of the curve of Fig. 2 was about 1 X 10~* 

 cm and after the electrodes became covered by sooty material, about 

 8 X 10~^ cm. The apparent striking field was decreased by activation 

 from 5 X 10^ to 0.6 X 10* volts/cm. When measurements were made at 

 250 volts, rather than 50 volts, the striking field in the active condition 

 was only slightly higher, 0.8 X lO"' volts/cm. Activation produces a 

 lowering of the apparent striking field, regardless of the value of the 

 applied voltage.* 



1.2 Arc Voltage 



The observed voltage across an arc at active palladium contacts agrees 

 in general with that of palladium cathode arcs, which is about 16 (Refer- 

 ence 4, Fig. 7 and Reference 2, Table II), whereas the arc voltage of 



* This apparent contradiction of the conclusion of F. E. Haworth^ is clarified 

 in Section 2.1. 



