POLYETHYLENE INSULATED TELEPHONE CABLE 



1253 



Table II — Comparative Data on 51-Pair 19-Gauge Cables 



* Computed from primary constants. 



constant. The low value of capacitance unbalance obtained with solid 

 polyethylene insulation is a result of the remarkable uniformity with 

 which this insulation is extruded. The value of 104 micro-microfarads 

 per 1500 feet for the capacitance unbalance to ground is on the average 

 only 0.4 per cent of the direct capacitance of either wire to ground. 



The other important factor in unbalance is the uniformity of the 

 twisting, that is, the extent to which wire and mate form symmetrical 

 helixes around the center line of the pair. Polyethylene-insulated pairs 

 appear to be better in this respect for reasons which are not very ob- 

 vious. The fact that the polyethylene forms firm tubes of insulation of 

 equal size on both conductors of the pair probably is a factor in achieving 

 this uniform twisting. 



The precision or accuracy with which the length of pair twist is 

 maintained is also better with the solid polyethylene insulated conduc- 

 tors. If an adequate number of different twist lengths is used in the 

 cable precision twisting is an advantage since the cross talk between 

 adjacent pairs is reduced over that which exists in the less precisely 

 twisted paper insulated cables. The cross talk between pairs with like 

 lengths of twist is kept to a low value by designing the cable so that 

 such pairs are widely separated. 



The carrier-frequency attenuation of polyethylene-insulated cable is 

 substantially lower than that of paper-insulated cable which has ap- 

 proximately the same voice-frequency attenuation. In the case of solid 

 polyethylene cable there are two reasons for this lower carrier-frequency 

 attenuation — the inductance is higher and the conductance is lower 

 than those in comparable paper-insulated cable. The higher inductance 



