268 BELL SYSTEM TECHNICAL JOURNAL 



tlian formerly, for reasons of plant economy. This trend thus increases 

 the severity of the cross-talk requirements. 



Unbalances in loaded circuits which contribute to noise due to 

 induction from power transmission and distribution circuits are 

 similar in nature to those contributing to cross-talk. The precautions 

 which are taken in the design, manufacture, and installation of loaded 

 circuits to reduce unbalances ha\e the effect, therefore, of reducing 

 both cross-talk and noise. 



V. Telegraphy Over Loaded Telephone Circuits 



It had been the practise in the Bell System, before the advent 

 of loading, to employ circuits for simultaneously transmitting tele- 

 phone and telegraph currents. Two methods were in general use, (1) 

 the composite system, in which each line wire of the telephone circuit 

 provided a telegraph channel with ground return, and (2) the simplex 

 system, in which the two conductors in parallel were used with a 

 ground return. 



It was very desirable to continue to superpose d-c. telegraph cur- 

 rents on telephone circuits after the introduction of loading. The 

 possible detrimental effects of the superposed telegraph and tele- 

 phone currents passing through the loading coils did not require 

 serious consideration so long as the circuits were relatively short, 

 since the magnetic modulation in the loading coil cores due to super- 

 posed hysteresis effects was sufficiently small to be negligible. As 

 the length of the loaded circuit was increased, the interaction between 

 the telegraph and telephone currents which has been designated in 

 an Institute paper" as "flutter," was aggravated and serious dis- 

 tortion of speech resulted. 



Measurem.ents of flutter effects obtained with the two grades of core 

 material then in use, viz., 65-permeability and 95-permeability wire, 

 showed the lower permeability core material to be substantially 

 better in this respect. This material has already been adopted for 

 the high efficiency loading coils used on the large gage toll cable 

 circuits, and for open wire lines used in spanning considerable dis- 

 tances. 



In order to obtain improved transmission over composited and 

 loaded Nos. 16 and 19 A.w.g. cables, the side circuit and phantom 

 loading coils for this grade of service were redesigned in 1913 to emplo>- 

 65-permeability cores working to the same over-all dimensions. In 



27 "Hysteresis Effects with \'arving Superposed Magnetizing Forces," W. Fondillcr 

 and W. H. Martin, Trans. A. I. E. E., Vol. 40, 1921, p. 443. 



