1376 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 



single contacts is a constant: 



Ps = constant (1) 



where Ps is defined as the number of opens per "contact operation." In 

 some cases, a certain transient period may precede the equilibrium char- 

 acteristic "Ps = constant characteristic." This transient period is 

 usually relatively short and is neglected in this analysis. 



(2) Clearing of opens on single-contacts by operation: If an open 

 single contact is allowed to operate mechanically, it is possible that it 

 will clear itself after a number of operations. As discussed in a later sec- 

 tion, the opens obtained are never identical in nature. They, instead, 

 have a certain statistical distribution which usually accounts for a wide 

 spread in their clearing rate. If, however, the operating conditions are 

 under control, the clearing characteristic of a set of contacts is found to 

 follow a well defined and reproducible statistical distribution. Fig. 2 

 shows an accumulative distribution curve for clearing opens produced 

 by cotton lint fibres.* The ordinate represents the fraction of the open 

 single contacts that clear after N operations as given by the abscissa. 

 In general, these relations have the following typical characteristics. The 

 first operation following the occurrence of the open is the most efficientf 

 single operation in clearing opens. It is usually responsible for clearing 

 10 to 30 per cent of the total number of opens. The subsequent opera- 

 tions are progressively less efficient and in general a certain fraction 



5 6 8 10 20 



NUMBER OF OPERATIONS, N 



30 40 50 60 80 100 



Fig. 2 — Distribution of clearing opens caused by lint. 



* This is one of the major causes of open contacts in central offices. These fibres 

 are usually in ribbon form of various configurations. 



t This apparent efficiency is only due to the presence of opens that are more 

 easy to clear than others. These will readily clear after one or a few operations. 



