OP EN- WIRE TELEPHONE LINES 



745 



C is such as to make the resistance component equal to the leakage 

 component. Because of the variation of the resistance and leakage 

 conductance with frequency, the ratio of L to C which gives minimum 

 attenuation evidently depends upon the frequency. At voice fre- 

 quencies the resistance component for open-wire circuits is, as a rule, 

 considerably larger than the leakage component, so that it is generally 

 possible to reduce the voice-frequency attenuation by inserting load- 

 ing coils, which increase the value of L and thus reduce the resistance 

 component at the expense of an increase in the leakage component. 

 The amount of reduction in attenuation obtainable by loading is 



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FREQUENCY-MLOCYCLES 



Fig. 9 — Voice-frequency attenuation of open-wire pairs equipped with DP 



insulators. 



evidently limited by the value of the leakage component and by 

 the additional resistance which is contributed by the loading coils. 

 For open-wire circuits at carrier frequencies the value of the leakage 

 component of attenuation is quite large in comparison with the resist- 

 ance component, and coil loading would, in general, be detrimental. 

 At the present time the use of loading on the open-wire circuits of the 

 Bell System has been practically abandoned. Owing to the importance 

 of other factors, especially the line crosstalk, it is ordinarily impracti- 

 cable to design the open-wire circuits to secure precisely the minimum 

 attenuation at the highest working frequency. In the carrier fre- 

 quency range, however, the wet weather attenuation of the pairs most 

 commonly used is not materially higher than the theoretical minimum. 



