INDUCTIVE LOADING FOR TELEPHONE FACILITIES 201 



The development of quadded cable and of phantom-group loading during 

 the period 1908-1910 introduced especially difficult requirements in the 

 control of crosstalk between the phantom circuits and their side circuits, 

 and to a lesser degree between the associated side circuits. Notwithstanding 

 further improvements in design and great care in manufacture, it became 

 necessary during the installation of the quadded cables to resort to the use 

 of special splicing procedures which reduced the total capacitance-unbalance 

 per loading section to satisfactory values. 



For nearly a decade it was the practice in the control of phantom-to-side 

 and side-to-side crosstalk to make the capacitance-unbalance test-splices at 

 seven approximately even-spaced intermediate splicing-points in each load- 

 ing section. By about 1919, further improvements in design and in manu- 

 facturing processes made it feasible to reduce the number of test splices per 

 loading section to 3. This practice is still general for quadded cables used 

 principally for two-wire repeatered facilities. In the period of very rapid 

 extension of the installation of long repeatered, loaded four-wire circuits 

 (1929-1931), it was found feasible to limit the number of test splices per 

 loading section to one, by using supplementary balancing-condensers to 

 reduce the high residual capacitance-unbalances on two-wire circuits, and 

 by using additional balancing-condensers at one end of each repeater section 

 on four- wire circuits, when required to obtain satisfactory low over-all 

 crosstalk. The one-way transmission in the individual, oppositely-bound, 

 paths of the four-wire circuits was a basic factor in making feasible these 

 field-adjustment simplifications, thereby reducing installation costs. 



14.2 Loading Coil Crosstalk 



Although the side circuit and phantom loading coils never had objec- 

 tionable design unbalances, it has always been difficult in manufacture to 

 realize the inherent symmetry of their design. The series inductance un- 

 balances have been the most troublesome accidental unbalances. In the early 

 days, reasonably satisfactory control of crosstalk between circuits in the 

 same phantom-group was obtained by care in manufacture, and by adjusting 

 the inductance unbalances to the nearest turn. 



A general, substantial, tightening of the crosstalk limits became necessary 

 when repeaters came into general use, because of the effect of the repeaters 

 in amplifying the unwanted crosstalk along with the wanted conversation, 

 and because of the great increases in circuit length that the repeaters made 

 feasible. During a period in the early 1920's, when intensive development 

 was under way to reduce the loading apparatus crosstalk, the over-all 

 repeater-section crosstalk was controlled by poling the unbalances of the 

 coils against the cable unbalances in the loading sections near the repeaters. 



The new development work, above referred to, included greater refine- 



