86 BELL SYSTEM TECHNICAL JOURNAL 



Cases arise, however, quite frequently where the relative circuit 

 impedances or the frequency make-up of the induction or both are such 

 that the noise contribution from the unbalances in the central office 

 equipment becomes important. Such cases usually involve long 

 subscriber or inter-office trunk circuits and particularly where sections 

 of open-wire construction are present. Values of the unbalances in 

 certain types of central office equipment are given on page 91 of 

 Volume I of the Engineering Reports of the Joint Subcommittee on 

 Development and Research. 



{b) Line Conductors 



Where the telephone line conductors are in open-wire, the induced 

 voltage between conductors (metallic-circuit) as well as along these 

 conductors (longitudinal-circuit) must be considered. The direct 

 metallic-circuit induction can be greatly reduced by systematic 

 transpositions in the telephone circuit. Due to the physical limita- 

 tions in a practical layout of telephone transpositions, the reduction 

 in metallic-circuit induction is, on the average, from 60 to 80 per cent 

 on non-pole pairs and about 90 per cent on pole-pairs. Transpositions 

 also tend to lessen the capacitance and inductance unbalances of the 

 two sides to ground and to other circuits, thereby reducing the effect 

 of the longitudinal-circuit induction on such unbalances. The im- 

 proved balance of the mutual impedances between the various tele- 

 phone conductors is, of course, distinctly beneficial in reducing cross- 

 talk and transpositions are generally used for limiting the crosstalk 

 where open-wire telephone circuits extend for substantial distances. 



The construction of telephone cables is such that there is inherently 

 very close spacing between the conductors and they are frequently 

 transposed due to the continuous twisting of the pairs in manufacture. 

 Due to this close spacing and frequent transposing there is practically 

 no voltage induced between the wires of a cable pair or quad (group of 

 four conductors). The unbalance to ground of the conductors of the 

 present type of cable is so small that it is not ordinarily a contributing 

 factor to noise induction. It may be noted, however, from Table B 

 that in cases where the central office unbalances are of importance, the 

 effect of shunt or series unbalances in the cables also needs consideration. 



The lead sheath of a telephone cable provides practically perfect 

 shielding against induction from power system voltages when it is 

 grounded at one or more points. The sheath likewise provides sub- 

 stantial magnetic shielding when it is grounded more or less continu- 

 ously as in underground construction or is grounded at both ends of 

 the aerial section or near both ends of an exposure. The degree of 

 magnetic shielding effected varies, depending on the size of the cable 



