522 BELL SYSTEM TECHNICAL JOURNAL 



good repeater balances being obtained and consequently restricted 

 the amplification which could be utilized on two-wire circuits. 



It was soon realized that even if the loading were made very uniform 

 tw^o-wire repeatered circuits would be restricted in their transmission 

 ranges, partly because of the unbalances encountered at repeaters 

 and partly because of the tendency of the circuits to crosstalk into 

 each other. Experiments were therefore begun utilizing the four-wire 

 circuit method. When four-wire circuits were first set up using the 

 large gauge loaded circuits between Boston and Washington great 

 difficulty was experienced in obtaining even reasonably uniform 

 attenuation at different frequencies. It then became apparent that 

 smoother (more uniform) circuits would be necessary. It also became 

 evident that higher cutoff frequency and higher velocity loading was 

 necessary, in order to widen the effective transmission band and reduce 

 delay distortion and echo effects. As a first step in this direction a 

 system of loading in which inductance coils of 175 millihenries on the 

 side circuits, spaced 6000 feet (1830 meters) apart, was introduced, 

 primarily for two-wdre circuits. This was commonly referred to as 

 medium-heavy loading, having a cutoff frequency of approximately 

 2800 cycles. Experiments on long four-wire circuits with this loading 

 which were specially set up for test confirmed the previous ideas as 

 to the seriousness of echo effects and delay distorton effects and made 

 it apparent that a much lighter weight and higher cutoff loading would 

 be necessary for great distances. Accordingly, a system of loading 

 known as H-44-25 (6000 feet — 1830 meters — spacing with 44-millihenry 

 coils on the sides and 25-millihenry coils on the phantoms) was 

 introduced. This was the beginning of modern long distance cable 

 circuits in America. 



Electrical Design of Toll Cable Circuits 



It is impracticable in a short paper such as this to deal fully with 

 all of the considerations which determine the electrical design of cable 

 circuits. There are set down here, however, the important character- 

 istics of the types of circuit which have been adopted for use in the 

 United States and the reasons why certain arrangements were selected. 

 Many specific designs which will meet the transmission objectives are, 

 of course, possible. The designs selected have been based upon the 

 aim of obtaining the desired results in the most economical manner, 

 including the advantage which comes from concentrating on a small 

 number of types of circuits, rather than on a larger number designed 

 to meet accurately the requirements of different types of situations. 

 In other countries where the ratios of costs for different parts of cable 



