270 BELL SYSTEM TECHNICAL JOURNAL 



VI. Recent Improvements in Loading for Exchange 

 Area Cables 



The developments discussed in the preceding sections were directed 

 to impro\ing and extending the range of long distance telephone 

 ser\ice. During the greater part of this period the loading standards 

 for exchange area trunk cables remained fixed. The first important 

 change occurred about 1916, when compressed powdered iron core 

 coils came into general use in place of the old standard wire core coils. 



In the period 1922-4, the use of new types of fine wire cables had 

 reached a point which required that certain changes be m>ade in the 

 old standard loading systems. Accordingly a new series of improved 

 loading systems having a considerably higher cutoff frequency than 

 the original standard systems, described in Table II, were developed. 



Cable Developments. Notable advances have been made in the 

 art of cable manufacture during the last decade or so, including the 

 standardization of 450-pair 19-A.w.g. cable, 900-pair 22-A.w.g. 

 cable, and 1,200-pair 24-A.w.g. cable, all contained in standard full 

 size sheaths (2^^ in. outside diameter). For each of the conductor 

 gages involved, each of these new maximum size cables has approxi- 

 mately 50 per cent, more conductors than the previous maximum 

 size ca:ble, typified by the old standard 300-pair No. 19-A.w g. or 

 600-pair No. 22-A.w.g. cables. 



The new^er types of cables have a smaller amount of paper insula- 

 tion on the individual conductors with a resultant increase in mutual 

 capacitance. 



About 1921 the methods of stranding the newer types of fine wire 

 cables (No. 22 and 24 A.w.g.) were changed in order to improve their 

 balance characteristics. These changes made the cables suitable 

 for the application of loading. 



The use of the old standard loading systems on the new types of 

 cables would have resulted in an objectionable impairment of quality, 

 due to the reduction of the cut-off frequency resulting from the in- 

 creased cable capacitance. Also the types of coils available were 

 more expensive than could be justified for permanent standards on 

 the low cost fine wire cables. Accordingly the development of new 

 loading systems and less expensive coils was undertaken. 



Determination of New Cut-Off Frequency Standard. The coil 

 design cost-balance study was taken up as one phase of a general 

 transmission-cost study of exchange area transmission, which also 

 included a theoretical investigation of cut-off frequency standards. 



In this work use was made of recent investigations of the effect 



