ARCING OF CONTACTS IN TELEPHONE SWITCHING SYSTEMS 



1503 



magnitude greater than the energy for inactive contacts. Contacts may 

 also be activated by inorganic films. Experiments on palladium and 

 silver contacts have shown* that glow discharge between contacts operat- 

 ing in air produced a second type of activation. Nitrides were formed on 

 the contact surfaces and the rainimum arcing current dropped to about 

 0.1 ampere for silver and 0.2 ampere for palladium. This effect was more 

 pronounced with silver contacts. 



In carrying out the measur\^ments of the minimum arcing current it 

 was observed that the arc wks generally interrupted by one of three 

 causes (1) the minimum arcing current was reached, (2) physical closure 

 of the contacts, and (3) shorting of the arc by a metal bridge formed 

 during the arc. A study of the bridge formation has shown that the 

 height of the bridge was a function of the arc energy. The bridge height 

 was measured by setting the zero separation point before and after the 

 arc. The difference gave the height of the bridge. This was plotted in 

 Fig. 9 against the measured arc energy. The height of the bridge in- 

 creased roughly with the cubic root of the arc energy up to energies of 



1.0 

 0.8 



0.6 



0.4 



^ 0.10 

 ^ 0.08 



0.04 



0.010 

 0.008 



^^ : ^^^:^ 



o ^ 



10 12 14 16 18 20 22 

 NUMBER OF CONTACT OPERATIONS 



2 4 6 



Fig. 8 — Increase in arc energy due to contact "Activation" 



24 26 28 30, 

 x103 



* This information was obtained from unpublished work of F. E. Haworth. 

 See also Reference 8 for a discussion of the effects of insulating films on arc initi- 



ation. 



