ACTIVATION OF ELECTRICAL CONTACTS BY ORGANIC VAPORS 773 



what vapor pressure is needed and how the minimiun pressure depends 

 upon the conditions of the test; for example, upon the electrical test 

 circuit. Some results of simple tests of this sort will be given before going 

 on to present anything more fundamental about what is happening when 

 contacts become active. 



Activation is produced, in general, by repeated operation of a pair of 

 contacts closing or breaking an electric circuit in air containing an organic 

 ^'apor. At the beginning of a test of this sort, oscilloscope traces look the 

 same as they would look in the absence of the vapor, but with continued 

 operation, arcs may begin to occur when there were initially no arcs, 

 or the durations of arcs may become greater. Activation is not produced 

 b}' simply allowing contacts to stand idle in an activating vapor even 

 for very long times, and even when the partial pressure of the vapor is 

 extremely high. Nor is activation produced unless currents are made or 

 broken. Operating contacts "dry" in a high pressure of an activating 

 vapor does indeed make them temporarily active,* but this condition 

 is unimportant from the standpoint of erosion. Under conditions that 

 are effective in producing acti\'ation, the number of operations required 

 before increased arcing can be detected is usually greater than 100, and 

 often greater than 10*. 



In general, noble metals can be activated by organic vapors and base 

 metals cannot. Vapors of unsaturated ring compounds produce activa- 

 tion and other organic vapors do not. Tests have been made upon the 

 metals Ag, Au, Cu, Pd and Pt which can be activated and upon Co, Cr 

 Fe, Mn, Mo, Ni, Sn, Ta, Ti and W which have not been activated. f The 

 vapors of nearly 50 organic compounds have been tested, about half of 

 them unsaturated ring compounds which produce activation, and about 

 half other compounds which do not. (These are listed, in part only, in 

 Reference 2, Table 1). A very large number of tests have been carried 

 out upon benzene, limonene and styrene. For these three compounds 

 the minimum partial pressures which just produce activation of silver 

 electrodes in air under certain standard conditions, and of platinum 

 electrodes in air, for which the results are the same as for sih'er, were 

 found to be respectively 0.1, 0.03 and 0.003 mm Hg. 



Some insight into activation is obtained by direct examination of 

 active contacts. Black soot can always be seen on active contacts, and 

 if they have been operating for some time in activating vapor, the 

 amount of soot may be great enough to produce a visible deposit under- 



* See the Section 7.3 on "Brown Deposit." 



t Ni and ]V have been activated in the presence of an organic vapor in a con- 

 tainer in which the pressure of air was reduced to 0.01 mm Hg, Reference 5, page 

 1090. 



