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THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



Part I — Electrical Effects 



1. OBSERVATIONS ON ACTIVATION 



Just how different the behavior of contacts can be in the clean or inacJ 

 tive condition, and in the active condition, is shown strikingly by the 

 oscilloscope traces of Fig. 1. Each trace represents a plot against time 

 of the voltage across a pair of protected relay contacts when the contacts 

 are pulled apart to break a current through an inductive load. The cir- 

 cuits used with the two pairs of palladimn contacts were identical, 

 the only difference of an}^ sort being in the conditions of the surfaces of 

 the contacts produced by exposure of the second pair of contacts to 

 organic vapor. In the trace of Fig. 1 (a) , the potential across the contacts 



If) 



60 



O 



> 



(0 50r 



o 

 > 



400 100 



TIME IN MICROSECONDS 



200 



300 



400 500 



Fig. 1 — Oscilloscope traces of the voltage across relay contacts breaking a 

 current of half an ampere through an inductive relay load. In each case a stand- 

 ard protective network of a 0.5 fii capacitor in series with 100 ohms is in parallel 

 with the contacts, (a) Clean or "inactive" contacts, with no observable arc. (b) 

 Active contacts, with a sustained arc lasting 400 microseconds. 



rises abruptly on break from zero to 50 volts, and then continues to 

 increase as the capacitor of the protective network is gradually charged 

 up; there is no arc or other discharge at the contacts. In the trace of 

 Fig. 1(b), the potential rises on break to about 14 volts and remains at 

 this value for 400 microseconds. This represents an electric arc that oc- 

 curred across the contacts, the 14 volts being the potential characteristic 

 of arcs over short distances at pallaium electrodes (Reference 2, Table 

 II). The energy dissipated at the contacts by this arc was about 25,000 

 ergs. 



A number of worth while experiments upon activation can be carried 

 out with no better method of measuring, or detecting, activation than 

 the observation of oscilloscope traces like those of Fig. 1, or correspond- 

 ing traces obtained from contacts discharging a small capacitor on closure 

 (Reference 2, Fig. 1). One can find what organic vapors produce activa- 

 tion, and what metals can be activated. This can be extended to discover 



