INDUCTIVE LOADING FOR TELEPHONE FACILITIES 171 



The new coils also were considerably better than the 95-permeability wire- 

 core coils in the following important features: 



(a) Their susceptibility to changes in inductance and effective resistance 

 during service intervals involving the superposition of steady d-c 

 signaling currents; 



(b) Their susceptibility to the transient magnetizing effects of superposed 

 composite-telegraph currents, i.e., "telegraph flutter". 



The relative performance characteristics, above described, resulted in the 

 "soft-iron dust" core coils superseding the standard 95-permeability wire- 

 core coils in the fields of use in which these older standard coils had been 

 used. As an important example, the original standard 508 coil, used princi- 

 pally for medium loading in exchange cables, was superseded in 1916 by the 

 574 coil, which remained standard for about a decade. 



The telegraph-flutter characteristics. Item (b) above, prevented the new 

 coils from being used generally in place of 65-permeability wire-core coils 

 on toll cables quads having all four wires composited for grounded telegraph 

 operation. However, for a few years there was a "compromise" practice of 

 combining "soft-iron dust" side circuit loading coils with 65-permeability 

 wire-core phantom loading coils in 19 and 16-gauge toll cable projects where 

 the needs for superposed grounded-telegraph operation could be satisfied 

 by compositing the phantoms, and the demands for repeatered facilities 

 could be met by luniting repeater operation to the side circuits. In this 

 special loading setup, the transmission distortion by "telegraph flutter" was 

 controlled in the phantoms, and was completely avoided in the side circuits 

 because the grounded telegraph currents, flowing in parallel through the 

 side circuit coil windings, neutralized each other's effect in magnetizing the 

 cores. With respect to regularity in circuit impedance-frequency characteris- 

 tics in relation to repeater gains, the high residual-inductance stability of 

 the soft-iron dust-core loading coils made them distinctly preferable to the 

 65-permeability wure-core coils in the repeatered side circuits. 



During 1917-1918, when the subsequently described work on improved 

 loading systems for long repeatered toll cables got well under way, theoretical 

 studies of the use of soft-iron dust core loading coils on such facilities dis- 

 closed seriously objectionable non-linear transmission distortion that had 

 not been bothersome on short circuits. This was due to the relatively large 

 hysteresis losses m the leading coil cores, which cause the effective resistances 

 of the coils and the circuit attenuation loss to mcrease appreciably in mag- 

 nitude as a function of line current ampUtude. The effects of these losses 

 are much more serious in the repeatered circuits, because of the larger line 

 currents, and because of the much greater circuit lengths. Since the hystere- 

 sis losses also vary in durect proportion to the telephone frequency , the result- 

 ant coil-resistance increments and attenuation increments are greater at 

 the high-speech-frequencies than at low frequencies. 



