58 BELL SYSTEM TECHNICAL JOURNAL 



of Fig. 23 and the simple "A" transient of Fig. 24, were all produced 

 under identical conditions in quick succession. The change in 

 characteristics from Fig. 18 to Fig. 24 was produced merely by a 

 reduction in the length of the connecting wire from 1100 ft. through 

 three intermediate stages to 10 ft., a 1000-ohm load relay being used. 

 This explains a puzzling effect noted with many contact materials. 

 With a supposedly identical circuit, the erosion will be small with 

 very short wires, increase rapidly as the wiring length increases, and 

 then decrease again becoming very small with very long wires. 



The "B" transient is more frequently observed with freshly filed 

 contacts, at high humidities, and with a rolling or wiping motion of 

 the contacts in opening. It is always found if the contacts are of 

 oxidized metal or operate in an oxygen atmosphere. In fact, there 

 seem to be good reasons for believing that its production is bound up 

 with the presence of oxygen on or in the surface of the active contact » 

 metal. 



It may be seen from the last series of oscillograms that if the circuit 

 and the conditions of the contact surfaces are just right, the "B" 

 transient is replaced by a much simpler and less stable type, the "A" 

 transient. It will occur usually when the wiring is short or the load 

 relay is of low impedance, with contacts which have been operated 

 until the original surface has been burned off and have not stood idle 

 more than a few minutes. It starts much as does the "B" type, but 

 after a dozen or a hundred sparkovers from about 350 volts, which 

 come much closer together in time than those of the "B" transient, 

 the voltage becomes steady at about 300 volts. This condition lasts 

 for perhaps 0.6 millisecond, then the voltage rises to about 400 or 

 450 volts and gradually reduces, reaching the battery voltage after 

 several milliseconds. A typical "A" type transient is shown in 

 Fig. 24. It is suggested as a hypothesis that, during the sparkover 

 stage, the oxygen is being exhausted from the surfaces of the current 

 carrying areas of the contacts by burning the metal and that when 

 this has been completed, a nitrogen gas glow discharge is formed and 

 maintained during the rest of the contact opening, if the supply of 

 energy from the load inductance through the line is rapid enough to 

 prevent the voltage from dropping below about 280 volts. 



The glow discharge phase of the "A" transient is unstable. If 

 transient voltages induced by the operation of relays in other circuits 

 reach the contact gap during the time that a sustained glow discharge 

 is attempting to form, its formation is interfered with and a mixed or 

 "B" type transient results. Occasionally, as illustrated in Fig. 22, 

 the glow discharge of the "A" transient has superposed on it a saw- 



