1248 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1953 



Fig. 2(a) — Cross section of expanded polyethylene insulation from 19 gauge 

 conductor — 35 per cent air. Magnified 75 times. 



ing a sample of insulation, with the conductor removed, on an analytical 

 balance. The inside and outside diameter of the cylinder of insulation 

 and the density of solid polyethylene are required to complete the de- 

 termination. 



The composite dielectric constant obviously varies with the degree 

 of expansion. In a coaxial configuration this effect is calculable from the 

 formula for the dielectric constant of a mixture,* the relation being 

 given by the following: 



g -- <p = y^ffJlJp) 



36 (€a + 2e) 



where < = composite dielectric constant 



Cp =» dielectric constant of polyethylene = 2.26 



€a = dielectric constant of added material, in this case 1 for air 



K- volume fraction = P5I2^r 



100 



