AKCIXG OF ELECTKICAL CONTACTS IN TELEPIIONK SWITCHING CIRCUITS 545 



a. Arc Iniiiaiion 



(1) The first initiatory electrons are produced by field emission. The 

 necessary field strength is largely dependent on cathode surface con- 

 ditions. It is highest for perfectly clean cathode surfaces and appreciably 

 lower in the presence of cathode surface films. ' ' This is probably due 

 to lower work functions or due to the presence of positive ions on a 

 cathode film causing local field intensification.^ (2) The field emission 

 electrons will travel to the anode where, to qualify for setting the second 

 step in arc initiation, should be able to produce, through evaporation, 

 some anode metal atoms* or possibly atoms of an adsorbed gas or a 

 surface film. (3) The potential drop across the contacts should exceed 

 the ionizing potential of the evaporated atoms to allow ionization by 

 electron collision. (4) Ions produced, on approaching the cathode, will 

 cause local fields high enough to produce electron avalanches. (5) the 

 above processes will rapidly multiply leading to the establishment of an 

 arc. 



h. The Established Arc 



One main characteristic of the short arc is its very high cathode current 

 density.! This high emission rate indicates that the short arc is not only 

 initiated hut also maintained by field emission.X^ Since the total voltage 

 drop across the arc is only of the order of 10 volts, the cathode drop 

 thickness should be very small compared to the total arc length. The 

 cathode drop is followed by the arc column or plasma which is a high 

 conduction medium with equal electron and ion densities, a small 

 potential drop and a relatively high neutral atom density. To maintain 

 the arc: (1) enough metal atoms should be produced to maintain the 

 necessary ionization medium, (2) ions lost by collection at the cathode, 

 by recombination and by lateral diffusion should be replaced by an 



* The arc may also he initiated without the assistance of tlie anode atoms or 

 ions*. The field emission current density at the cathode in this case, was found 

 to reach a critical value before the arc is initiated. It is thought' that at this 

 current density the emission spot can attain its melting point through resistive 

 heating. The cathode in this case will furnish the necessary metal atoms for the 

 subsequent steps of arc initiation. 



t Recent measurements by the author obtained from arc tracks on Pd contacts 

 produced by short duration conslanl current arcs indicated current densities as 

 high as 50 X 10^ amp/cm^. 



X Paper by P. Kislink to be published in the Journal of Applied Physics. 



§ Recent analytic considerations, to be j)ublished by the author, indicate that 

 in such arcs the current density should be dependent on the work function of the 

 cathode material as well as on the product "pressure X sejjaration" in the arc. 

 For instance, for work functions of 2 and 5 volts, our calculations show that the 

 minimum current densities are, respectively, 5 X 10^ and 1.4 X 10' amp/cm^. 



