54 



BELL SYSTEM TECHNICAL JOURNAL 



In many places the character of the country is such that under- 

 ground construction would be very expensive. In such cases, and 

 in other cases where it seems desirable, aerial toll cable construction 

 has been used extensively in the United States. With this type of 

 construction the cable is suspended from a steel messenger wire sup- 

 ported on poles. Figure 43 shows typical aerial cable construction, 

 including a loading point, the pots of loading coils being supported 

 on an angle iron pole fixture. 



Long circuits in toll cables have some extremely interesting elec- 

 trical characteristics. Figure 44 shows the net transmission charac- 



U1 



_l 

 m 



O 

 u 



Q 



— llJ 

 > 



UJ UJ 



Q -I 



o 



>- 

 o 



o 

 o 

 o 



400 800 1200 1600 2000 



CYCLES PER SECOND 



Fig. 44. 



2400 



2800 



3200 



teristic over a range of frequencies of a New York-Chicago toll cable 

 circuit 1,380 kilometers in length. It will be seen that the voice 

 frequencies are transmitted with nearly the same net efficiency over 

 a sufficiently wide band to give a high grade of telephone transmission. 

 The net characteristic indicated, however, is obtained by almost 

 wholly neutralizing with telephone repeaters the very large trans- 

 mission loss in the circuit. The New York-Chicago circuit, for ex- 

 ample, would have an attenuation loss at 1,000 cycles of about 470 db, 

 which means that without amplification the ratio of output power at 

 one end to input power at the other end of the circuit would be 10"'*^ 

 The combined gain of the 19 telephone repeaters in the circuit is 

 about 461 db, giving about 9 db net equivalent, lender these condi- 

 tions, it is evident that a careful regulation of the circuit is essential. 

 For example, variations in the temperature of a circuit in the course 

 of a day could make as much as 30 db or 1,000 fold difference in the 



