ACTIVATION OF ELECTRICAL CONTACTS BY ORGANIC VAPORS 785 



density necessary for the low voltage field emission arc, does one observe 

 the high and erratic arc voltage characteristic of carbon. 



2.3 Minimum Arc Current 



Values of minimum arc current for carbon electrodes have already 

 been published. They are of the order of 0.02 to 0.06 ampere and agree 

 fairl}^ well with measurements of minimum arc current for very active 

 metal contacts (Reference 2, Table V) . The very low value of the mini- 

 mum arc current for carbon, either in solid form or dispensed upon the 

 surfaces of active contacts, is related to the low electrical and thermal 

 conductivities of carbon. These low conductivities permit explosion of 

 carbon particles on the cathode by currents too small to vaporize any 

 metal. It has already been pointed out that it is this very low value of 

 minimum arc current which accounts for the greatly enhanced energy 

 that is dissipated at active relay contacts. 



From the low value of minimum arc current for active surfaces, one 

 concludes that near its end an active arc is always located at a fresh 

 point on the electrode surfaces, one from which carbon was not burned 

 off earlier in the life of the arc. It had already been concluded from oc- 

 casional high values of arc voltage near the end of an active arc that 

 this is sometimes true, but the minimum arc current values extend this 

 earlier conclusion to indicate that it is always so. An active arc cannot 

 remain in a fixed position as does an inactive anode arc (For example, 

 Reference 4, Fig. 1). The implication is thus suggested that any arc 

 between active palladium contacts is a cathode arc. Further presumptive 

 evidence for this is, of course, furnished by the very much greater elec- 

 trode separation in the case of active arcs; it is well known'^ that large 

 distances favor cathode arcs, because at great distances the anode cannot 

 be efficiently heated by electron bombardment. 



The interpretation of minimum arc current of active cathode arcs to 

 which we have been led can be written down in words, but we have not 

 succeeded in any quantitative formulation. It is well known that every 

 cathode arc is made up of a great number of small arcs moving continu- 

 ally over the electrode surfaces and exploding one point, or one particle 

 after another on the cathode.^' * In the case of an active arc, the end 

 comes when the current gets so low that it will no longer explode a car- 

 bon particle, or when no suitable particles are available.* The much 



* This is a necessary criterion for the end of an active arc only in the case of 

 very short arcs. For electrodes that are being pulled apart to break a current larger 

 than the minimum arc current, an arc will, of course, finally fail because of the 

 great electrode separation, even though the current is al)ove the minimum arc 

 current, as in the final failure of the arc in the oscilloscope trace of Fig. 1(b). For 

 inactive anode arcs the minimum arc current arises in a quite different waj' and 

 has been interpreted in fairly satisfactory quantitative fashion. '^ 



