790 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



zero, and for an arc lasting longer than this time, there may be cathode 

 loss and actual net gain by the anode. No such balancing effect is possible 

 for palladium. 



2.4{c) Anode Arcs and Cathode Arcs. The model of an active cathode 

 arc to which we have been led seems fairly clear and rather well estab- 

 lished, but the model of an active anode arc is more poorly defined. From 

 electron micrographs of the damage done to the cathode by an arc of 

 the cathode type (Reference 4, Fig. 3), it is kno\Mi that an arc of this 

 type is intermittent, striking over and over again. In an active arc of the 

 cathode type, a carbon particle on the cathode is blo^vn up each time 

 the arc strikes, but always there is metal vaporized from the cathode at 

 the site of the particle and the amount of vaporized cathode metal is 

 greater than the amount of vaporized carbon, so that the arc is an arc 

 in metal vapor. 



We know less of an active anode arc, and it may well be that some 

 experiments described above seem to imply a model which is not con- 

 sistent with other observations. The facts that we know are, that at a 

 lightly carbonized silver surface an arc strikes at an electrode separation 

 much greater than the separation at which it would strike if there were 

 no surface carbon, that the resulting arc produces loss of metal pre- 

 dominantly from the anode, and finally that the minimum arc current 

 is very low. The arc is a true anode arc by our implied definition of such 

 an arc, yet it is certainly an active arc. When the arc current is high, a 

 crater is being produced on the anode as in the case of an inactive anode 

 arc, and also in the case of an active anode arc at a surface on which a 

 few carbon particles of diameters not greater than 2.5 X 10~* cm have 

 been dusted. When the current becomes too low, or is too long sustained, 

 one presumes that the arc is extinguished as in the case of inactive anode 

 arcs." It may then restrike at another carbon particle. One speculates 

 that an anode arc is intermittent when the arc current is very low, being 

 initiated over and over again as are cathode arcs throughout their lives. 

 A carbon particle is exploded repeatedly on the cathode. Yet, because 

 the separation is less ^than the critical distance, at each re-ignition of 

 the arc, metal vapor is derived from the anode rather than from the 

 cathode, and possibly the over-all anode erosion results in a single anode 

 pit produced when the current was sufficient 1}^ high, plus an array of 

 very small anode pits formed while the current was small and intermit- 

 tent. This model must be regarded as a plausible speculation without 

 support in direct observation. The existence of the active anode arc is 

 well established although the course of such an arc is speculative. 



Some insight into the reason for the existence of a critical electrode 



