1380 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1954 



every half-open persists indefinitely, and is given by (P<)oo = H^s . In 

 other words, hij replacing single contacts by twin contacts the frequency of 

 failures is decreased by only one third. In practice, however, /^ is rarely 

 that low and under the wide variety of central office conditions it may 

 range between 0.85 and 0.98. This corresponds to a range of performance 

 of Pt = 0.23 Ps to 0.038 Ps or the frequency of twin-contact failures is 

 about yiioV2Q that of single contacts. 



CLEARING AN OPEN BY MECHANICAL OPERATION 



IntrodvA^tion 



Fig. 3 shows a diagrammatic sketch of the contact area with spherical 

 particles preventing metallic contact. The surface irregularity has a 

 slope <p and the particle diameter is d. It is evident that the particle will 

 produce an open if it falls within a limiting radius r^ = d/ip. The area 

 ■within fo is called the "open zone." For a particle at a radius r within 

 the open zone, mechanical wipe w mil tend to displace the particle at an 

 angle 0, ^ ^ ^ 27r. The open is cleared when the particle is displaced 

 a distance X sufficient to drive it out of the open zone. For unidirectional 



