TWELVE-CHANNEL OPEN-WIRE CARRIER SYSTEM 



381 



"Tree" wire, a special line wire with abrasion-resistant insulation, 

 has been used on open-wire lines for many years in places where the 

 lines were exposed to tree branches. During line tests in Florida, 

 another use for tree wire was found where the open-wire line, along a 

 causeway or bridge, is subject to fouling by fishing tackle. Curve C of 

 Fig. 11 shows what a half-mile or so of this tree wire, supplemented by 



50 75 100 



FREQUENCY- KILOCYCLES PER SECOND 



Fig. 12 — Attenuation of toll entrance cable pairs. 



several sections of 165-mil wire at railroad and power-line crossings, 

 can do to the impedance of a 128-mil pair. To reduce the irregularities 

 a new type of insulated line wire of smaller diameter and with thinner 

 insulation was developed. This wire has about the same impedance 

 characteristic as the line wire. 



Intermediate Cable Treatment 



When open-wire lines have to be placed underground to pass 

 through towns or to cross natural barriers such as rivers which cannot 



