CONTACT PHENOMENA IN TELEPHONE SWITCHING CIRCUITS 57 



charges slowly back through the load relay to the battery voltage. 

 The peak voltage reached may be as high as 2000 volts. 



The principal characteristics of the "B" discharge can be produced 

 by a simple experiment which does not use a load relay, The transient 

 is not dependent on a load inductance, but only on a source of voltage 

 which will charge a wire at a sufficiently rapid (but not too rapid) rate 

 while a pair of contacts, which initially ground the wire, are separating. 

 If a wire about 100 ft. long is connected to a source of somewhat more 

 than 350 volts through a resistance of from 5000 to 20,000 ohms, and is 

 also grounded by a contact at one end, a transient is produced when 

 the contacts open which shows the characteristics of a "B" type 

 transient except the final dying away of the voltage to 50 volts. 



It must not be understood that every spark transient is purely of 



either the "A" or the " B " type. It is very common for the "A" type 



• transient to break down into the "B" type and less often the "B" 



transient establishes the gas glow discharge for a brief period in the 



middle of the sparkovers. 



A "mixed" transient is shown in its entire duration in Fig. 17. 

 Here, after a group of sparkovers, a period in which the voltage is 

 maintained steadily at about 300 volts for about 0.0002 second inter- 

 venes, and is followed by more sparkovers from considerably higher 

 voltages. In order to produce this transient the length of the line 

 wire was reduced to 10 ft., at which length and with a 1000-ohm load 

 relay the tendency is to produce intermittent groups of "A" or "B" 

 transients, interspersed with the mixed type shown, when the relay is 

 operated frequently. 



The number of sparkovers in each "B" transient varies with the 

 circuit conditions. As many as a thousand may be found with a load 

 consisting of a number of relays in parallel on a wire of moderate 

 length and as few as one in the limiting case. 



While the occurrence of the "B" transient is favored by long line 

 wires and high impedance relay loads, beyond a certain length which 

 with telephone relays and wiring is from 300 to 2000 ft. (the longer 

 lengths being associated with the lower impedance relays) no spark- 

 overs at all occur. The voltage build-up is so slow that the sparkover 

 potential is not reached at any time during contact opening and the 

 contacts may be said to be protected by the line wire. The series of 

 voltage oscillograms. Figs. 18 to 24 inclusive, shows the change from a 

 smooth transient with no sparkovers through the "B" type with an 

 increasing number of sparkovers to the final "A" type. The "A" 

 type transient of Fig. 22, which has superposed on the 300-volt gas 

 glow discharge stage a rela.xation type of oscillation, the "B " transient 



