798 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



becoming constant. One such set of measurements is plotted as Fig. 10. 

 In this experiment the striking voltage was 232 and the energy in each 

 arc 1,250 ergs. The final slope of the curve of Fig. 10 corresponds to the 

 production of 4.5 X lO"^^ gm of carbon per arc — 3.5 X 10~^^ gm/erg, 

 1.8 X 10^ atoms/erg — which is 0.04 carbon atom for every electron 

 flowing in the arc, or the decomposition of 3.7 X 10^^ benzene molecules 

 per arc, 3 X 10^ molecules per erg, or 70 X 10"'* molecule per electron. 

 The lower slope, before the break in the curve, represents the decompo- 

 sition of 1.1 X 10^^ molecules of benzene per arc, or the production of 

 5 X 10^ carbon atoms per erg. 



From continuous oscilloscopic observations it was found that the 

 contacts were inactive up to the point where the slope of the curve in- 

 creased. Here they were slightly active, and beyond this point they were 

 fully active, exhibiting the usual apparent low striking field and low 

 minimum arc current. The amount of carbon required to make the con- 

 tacts fully active was about 5 X 10~^ gm (2.5 X 10^^ atoms) which, if it 

 were in a single spherical speck, would have a diameter of 3.5 X 10~* 

 cm. Such a speck can be seen quite easily with the naked eye, although 

 an actual deposit of this volume probably could not be seen without a 

 microscope because of its dispersed state. 



5. SURFACE ADSORPTION 



5.1 Benzene Molecules on Contact Surfaces 



In an early experiment, the rate of formation of carbon (from meas- 

 ured rate of evolution of H2) had been found to be independent of ben- 

 zene pressure down to the lowest pressure tested, which was of the order 

 of 10~^ mm Hg. For this reason it was unnecessary to mention absolute 

 pressures in describing the above tests. This lack of dependence on pres- 

 sure suggests strongly that benzene had been adsorbed on the electrode 

 surfaces and decomposed there, rather than in the space between the 

 electrodes, and that the lowest pressure tested was sufficiently high to 

 keep the surfaces completely covered. This tentative conclusion is con- 

 firmed by other considerations given below. 



At the pressure of 10~^ mm Hg and an electrode separation of 10~^ 

 cm, one calculates that only one electron in 3 X 10* can collide with a 

 benzene molecule in the space between the electrodes in the experiment 

 of Fig. 10. The discrepancy between the measured decomposition (70 X 

 10~* benzene molecule per electron) and the possible frequency of col- 

 lision (0.3 X 10~*) is proof that most of the carbon responsible for activa- 

 tion comes from benzene adsorbed on electrode surfaces rather than 

 from molecules in the space between them. 



