1254 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



5 10 15 20 25 30 35 40 45 50 



PER CENT AIR IN POLYETHYLENE INSULATION 



Fig. 6— Inherent dielectric strength of expanded polyethylene versus degree 

 of expansion. 19-gauge 0.064-inch D.O.D. conductors. Short samples immersed in 

 water. 



is a result of the greater separation between the wires of a pair and the 

 lower conductance is a result of the lower power factor of polyethylene 

 compared to that of paper. The voice frequency attenuation is relatively 

 independent of inductance and conductance. 



While the electrical characteristics of polyethylene cables are superior 

 to those of paper cables, the higher first cost of cables insulated with 

 solid polyethylene has been a deterrent to their widespread use. This 

 higher first cost is inherent in solid polyethylene because, in addition 

 to the higher cost of polyethylene as compared to paper, the cables 

 must be larger for the same voice frequency attenuation. It will be 

 noted that the 51 -pair Trout Lake-St. Ignace cable is 15 per cent larger 

 in diameter than the comparable paper cable. The larger size is necessary 

 because of the higher effective dielectric constant which is approximately 

 1.80 in solid polyethylene cable as compared to 1.60 in a typical paper 

 cable. The effective dielectric constant is higher in the case of solid 

 polyethylene insulation because of the lesser amount of air space which 

 can be incorporated in the dielectric between wires. 



As was illustrated in Fig. 3, the dielectric constant can be decreased 

 by expanding the polyethylene. A value as low as 1.40 has been at- 

 tained experimentally. The effective dielectric constant of the Grand- 



