462 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



tubes fired simulatneously will then travel through this unstable region 

 exactly together, an inductive element is used in the common load cir- 

 cuit. This increases the time interval required to traverse the unstable 

 region thereby permitting differences between tubes to result in lockout. 



In each junctor a five-element cold-cathode gas tube is used for path 

 detection and selection. One control element of this tube is marked from 

 the line side and other control element from the trunk side of the junctor 

 if this junctor is usable in the call being set-up. The main anode is con- 

 nected, together with those of the other junctors of the same line frame, 

 in a lockout circuit so that only the gas tube in one junctor can conduct 

 in its main gap. The junctor in which the gas tube does conduct in the 

 main gap is the selected junctor and the switching network path associ- 

 ated with it is the selected path. Assume that junctor 2 is so selected. It 

 first shorts out the resistors in its — 105-volt supply leads. This permits 

 a higher value of current to flow through the gas diodes along the selected 

 path and causes the operation of the reed contacts associated with the 

 crosspoint relay windings which are in series with the diodes. The control 

 lead contact at each of these crosspoints, as shown along the selected 

 path in Fig. 10, shorts out the gas diodes. With the diodes shorted out 

 a further increase in the current operates relays in series with this con- 

 trol lead path in the line and junctor circuits. These relays cause the 

 — 105-volt supplies in the associated junctor, junctor 2 in this case, 

 to be replaced by the —24- volt sources and the +80- volt marks on the 

 line and trunk terminals to be replaced by ground. This shift of power 

 sources permits the gas diodes along paths marked but not selected for 

 this call to extinguish but holds at a low power level the crosspoint 

 relays along the selected path. With all diodes extinguished the switching 

 network is ready for the next path selection operation. Removal of the 

 ground at the trunk end of an established connection, at the end of 

 conversation, results in complete release of the associated operated 

 crosspoints and junctor. 



With a central office traffic rate during busy hours of 50,000 calls per 

 hour, 50 milliseconds is the maximum allowable holding time for a single 

 common control ciruit at 70 per cent usage. A single control circuit, 

 even during its busiest periods, should not be in use more than about 70 

 per cent of the time. If the usage is increased beyond this point the 

 delays which other circuits encounter in attempting to use the common 

 control circuit increase very rapidly. This produces the same effect as 

 increased control circuit holding time. 



The holding time of the control circuit for the switching network 

 determines the traffic capacity of the switching arrangement if only a 



