574 



BELL SYSTEM TECHNICAL JOURNAL 



losses in the insulators the attenuation increases steadily with fre- 

 quency. Unfortunately the losses at the insulators are not constant 

 and they increase greatly with the presence of moisture. This brings 

 about an increase in attenuation in rainy weather. Fog, sleet and 

 wet snow may greatly increase these attenuation changes. There is 

 also a lesser source of variation due to temperature change and its 

 effect on wire resistance. 



If care is not observed, the carrier currents may be interfered with 

 on the line circuits by crosstalk from other carrier systems and by 

 miscellaneous currents which enter the circuit by induction from the 

 outside. These latter manifest themselves as noise in the carrier 

 channels. This makes it essential to use only the metallic circuit, 

 i.e., two wires well balanced to ground for transmitting the carrier 

 currents. The balance to ground must be maintained at a high degree 

 by frequent transpositions in the wires. Even with these precautions 

 unavoidable residual unbalances may permit a certain amount of 

 interference to appear. The final remedy is to insure that the relations 

 between the circuit length and the apparatus gains are properly 

 considered in order that the speech currents may have ample margin 

 above the noise currents at all points in the circuit. 



In the matter of crosstalk between systems closely adjacent on 

 the same line the situation is alleviated by providing two frequency 

 allocations. (See Figure 5.) These are "staggered" with respect to 



5 10 15 20 2b 30 



FREQUENCY -KILOCYCLES 

 Figure 5 — Frequency allocations of type "C" system 



each other, so that a system installed on one pair using the so-called 

 "N" frequency allocation has less crosstalk to and from a system 

 installed and operating on an adjacent pair and using the so-called 

 "S" frequency allocation than would be the case if both systems 

 employed the same allocation. The maximum upper frequency 

 required is raised only slightly by this arrangement. 



Repeaters. Rei)eaters must be employed when the distance exceeds 



