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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



1800 volts) just before the parts touch. The arc initiates itself when 

 the gap has been reduced to a few thousandths of an inch. A portion of 

 the abutting surfaces of both the wire and the contact base metal 

 (normally 0.005 inch to 0.010 inch) are melted and expelled in liquid and 

 gaseous states before the molten surfaces are forced together. The arc is 

 extinguished when it can no longer melt and expel metal to maintain 

 a gap. Under good operating conditions it persists from 0.1 to 0.4 milli- 

 seconds. Nearly all of the heated metal is expelled from the joint during 

 the welding operation as illustrated by Fig. 19. This micrograph of a 

 typical sectioned percussion weld shows only a 0.001 inch to 0.002 inch 

 thick layer which was melted or heated sufficiently to change the struc- 

 ture. 



A small stream of compressed air is directed into the weld area to 

 remove gaseous, possibly ionized, arc products so that they will not 



Fig. 21 — Schematic of percussion welder circuit. 



