ACTIVATION OF ELECTRICAL CONTACTS BY ORGANIC VAPORS 799 



One gets some insight into the adsorbed films responsible for activa- 

 tion from estimates of the cross-section of an arc and of the amount of 

 benzene adsorbed in a monolayer over an area of this size. A reasonable 

 estimate of the number of molecules in a monolayer of benzene is 7 X 

 10^^ cm~2 (Ref, 16), or 14 X 10^^ cm"^ taking into account the two 

 electrodes. Estimates of cross-sectional size have been published for 

 ! anode arcs, but for cathode arcs the areas are quite different. Since all 

 I of the arcs after a surface has become active are certainly cathode arcs, 

 ! our first concern is with the cross-sectional areas of cathode arcs. It has 

 been observed that the over-all area of the cathode markings made by 

 inactive cathode arcs increases somewhat less rapidly than linearly with 

 total arc energy, and seems to be independent of arc current and arc 

 duration except as they influence the total energy. In one series of ex- 

 periments, the areas observed (Ref. 10) for low energy arcs corresponded 

 to somewhat less than 10^ ergs/cm-, and to somewhat more than this 

 value for high energy arcs. Assuming for an average value 10^ ergs/cm^, 

 we obtain 1.2 X 10~'* cm^ for the area of the arcs of the curve of Fig. 10.* 

 This area should have adsorbed on it 1.7 X 10" benzene molecules. The 

 observed rate of decomposition is 3.0 X 10^ benzene molecules per erg 

 or 3.7 X 10" molecules per arc. Looking at photographs such as those 

 of Fig. 7, one does not feel at all confident that all of the surface in the 

 over-all area of the arc ever became hot enough to decompose benzene. 

 If all of it did become hot enough, the surface must, on the average, have 

 been covered by 2 layers of molecules, and if all of the surface did not 

 become sufficiently hot, by more than two layers. For lower energy arcs, 

 when the number of benzene molecules decomposed per erg is appreci- 

 ably greater, it is natural to assume that the surface must, on the aver- 

 age, be covered by a still deeper layer of benzene. At least part of the 

 difference between the estimated thicknesses of the layers of benzene 

 molecules for high energy arcs and for low energy arcs can, however, 

 be attributed to the fact that the energy per square centimeter increases 

 with increasing energy, 10^ ergs/cm- being only an average value. The 

 data indicate only that the adsorbed benzene layer is several (greater 

 than 2) molecules thick. 



The observed expression M = KE'^i^ of the above section, relating 

 amount of carbon formed M to total arc energy E, can be accounted for 

 if, in the particular experiment in which this relation was found, the 

 over-all arc area increased with the f power of the energy. In various tests 



* One should note that the energy density measurements were made upon clean 

 surface or inactive cathode arcs, but are being applied here to active cathode arcs. 

 Some justification for this is afforded by the fact that the active 50 erg cathode 

 arc of Fig. 7(b) had an over-all area of about 3 X 10~^ cm^ giving for the energy 

 density 1.5 X 10' ergs/cm^. 



