204 BELL SYSTEM TECHNICAL JOURNAL 



example, for horizontally adjacent pairs. The r.m.s. of the sag 

 differences in a representative number of spans was also determined 

 for the two pairs of each type of combination. The two r.m.s. values 

 for any particular type of pair combination were called R and r. The 

 ratio of i? to r gave a constant k for estimating R from a known value 

 of r and for Lo, the particular length of line tested. For other line 



lengths, R is estimated from the expression R = kryl — • Having 



computed R, the chance of the crosstalk for any pair combination in a 

 long line lying in a given range may be estimated by probability 

 methods. 



The second method of studying sag differences is more precise 

 although much more laborious. The change in crosstalk due to 

 introducing sag differences in but two spans is determined. The 

 poles are specially guyed to make it possible to adjust all the wires in 

 these spans to have practically the same sag. Turnbuckles are 

 installed at the ends of the two-span interval for this purpose. At 

 the center pole the wires were supported so as to slip readily and 

 equalize the sag in the two spans. 



The phase and magnitude of the crosstalk is first measured for all 

 pair combinations with all wires at normal sag. The wires are termi- 

 nated in the same way as in the measurements of crosstalk coefficients. 

 From sag measurements on actual lines, a set of unequal sag values 

 for all the wires is then selected by probability methods and the 

 crosstalk remeasured. The vector difference between the values of 

 crosstalk before and after introducing unequal sags is then determined. 

 This process is repeated a large number of times in order to cover the 

 range of sag conditions encountered in practice. An r.m.s. value of 

 the change in crosstalk due to sag difference is then determined for 

 each pair combination and related to the r.m.s. sag difference per pair. 

 This permits the probable crosstalk in a long line to be estimated and 

 the importance of sag difference crosstalk to be determined. The 

 two methods of study were found to be in general agreement. The 

 second method has been extensively used to study proposed new wire 

 configurations. 



Drop Bracket Transpositions 



An ideal transposition would cross the two sides of a circuit in an 

 infinitesimally small distance, there being no displacement of the 

 wires from their normal positions on either side of the transposition. 

 The point-type transposition indicated by Fig. 21 is close enough to 

 the ideal for practical purposes. Its deviation from the ideal requires 

 little consideration in transposition design. To avoid cutting the 



