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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



Table I — Effect of Carbon Particles upon 

 Striking Distance 



through agitated carbon dust and collecting the particles that had been 

 carried upward for a considerable distance in the air stream. The time 

 of deposition of these particles upon the smooth surface was adj usted to 

 give an average distance between particles of about 10 times their di- 

 ameters. The smooth palladium surface with a fairly uniform, but sparse 

 covering of carbon particles was made the cathode in measurements of 

 striking distance by the oscilloscope method, Section 1.1. For a particular 

 size of particle, 100 measurements were made of striking distance, each 

 measurement at a different point on the surface, so as not to include any 

 measurement of striking distance at a place on the surface where the 

 original particles had already been burned off.* Table I gives the ranges 

 of particle size as found microscopically and the corresponding average 

 measured values of striking distance. The increase of striking distance 

 was just equal to the particle size. At each arc, a particle was destroyed 

 so that the time to closure measured on the oscilloscope corresponded, 

 not to the true striking distance, but to the distance from the anode to 

 the cathode surface upon which the particle rested. The electric field at 

 which the arc struck was very much higher than the calculated \'alues 

 of the third column of Table I, and was not significantly different from 

 the striking field for inactive surfaces. 



In the fourth experiment, the striking field was measured between 

 electrodes of solid carbon. One of these was mounted upon a cantilever 

 bar in such a way that it could be moved through extremely small meas- 

 ured distances by pushing on the end of the cantilever bar using a mi- 

 crometer screw (Reference 4, page 33). The zero point was found by 

 touching the contacts through a high resistance galvanometer circuit; 

 then the contacts were separated and the striking distance found after 

 applying the voltage. Measurements made in this way by M. M. Atalla 

 (Reference 11, Table I) have given, for the striking field for carbon elec- 



* A correct measure of striking distance is obtained only when the arc energy 

 is sufficient to burn up the carbon particles completely. Xo appreciable mound of 

 metal is thrown up to falsify the distance measurement, because the arcs are of 

 the cathode type, see Section 2.4(a). 



