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 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As an important example, the original standard 508 coil, used princi- pally for medium loading in exchange cables, was superseded in 1916 by the 574 coil, which remained standard for about a decade.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 limiting operation to the side circuits. In this special loading setup, the transmission 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 wire-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 in the loading coil cores, which cause the effective resistances of the coils and the circuit attenuation loss to increase appreciably in mag- nitude as a function of line current amplitude. 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 direct 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.