188 



BELL SYSTEM TECHNICAL JOURNAL 



Discussion of Test Results 



In order to study the variation with frequency of the mutual in- 

 ductance between cable circuits, measurements were made on various 

 combinations of pairs in a 55-foot length of No. 19 A.W.G. toll cable. 

 To obtain information on the performance of the measuring apparatus, 

 measurements were also made on the calculable case of two non-twisted 

 pairs, six feet in length (approximately). Various separations between 

 the two wires of a pair and three different wire gauges were used to 

 show the change in mutual inductance for various degrees of proximity 

 effect. 



The measurements were made with a "crosstalk bridge" or ad- 

 mittance unbalance measuring set which permits the measurement of 

 crosstalk in both phase and magnitude. Although the mutual in- 

 ductance between two pairs may be determined from either near-end 

 or far-end crosstalk tests, it was found that greater accuracy in Mi 

 could be obtained from far-end tests. The computation of Mb from 

 near-end tests involves two terms of opposite sign and of nearly the 

 same magnitude. Consequently a small error in the reading of the 

 crosstalk bridge may result in a considerably greater error in Mb- 



The results of the tests on the six-foot non-twisted pairs are shown 

 in Figs. 3 to 9. The data cover a range of 1 to 1000 kilocycles. 



In Figs. 3 and 4 the variation with frequency of Ma and Mb is shown 



0.12 

 O.ll 



§ 0.08 



o 



2 0.07 



z 



uj 0.06 



u 



< 0.05 



\- 



o 



g0.04 



z 



_i 0.03 



< 



^ 0.02 



^ 0.01 







-0.01 



PAIR I PAIR 2 



O O O O 



U-t*K-r*Ur*l 



0.075" 



NO. 20 GAUGE WIRES 



76 INCH LENGTH 



/ 



/ 



y 



\ 



V 



-IOM|o 



3 4 5 10 20 30 50 100 200 300 500 1000 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 3 — Mutual inductance between pairs of parallel wires. 



