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BELL SYSTEM TECHNICAL JOURNAL 



of those molecules. These polarized molecules are called dipoles and when 

 an electric field is applied the dipole axes tend to line up in the direction of 

 the applied field. It is probable that for a combination dielectric such as 

 the paper and air in cables with possible traces of moisture, in spite of oven- 

 drying, the dipoles constitute only part of the charges. The frequency also 

 is too low, in most of the data, to emphasize the effects due to dipoles. The 

 paper-air combination introduces another slowing up of the polarization 

 process on account of interfacial polarization. Maxwell showed that if the 

 dielectric in a condenser consisted of two layers of materials having different 

 constants, the capacity depends upon the charging time because of time 

 required in charging the interface between the two dielectrics. For a-c. 

 this means a decreasing capacity with increasing frequency and, since there 



.0626 



.0618 



5 .0610 



^ .0602 



0594 



.0586 



10 



20 



30 



40 



50 



60 



70 



80 



100 



FREQUENCY -KILOCYCLES 

 Fig. 8 — Capacitance per mile vs. frequenc}' — 19 gauge pairs 



are effectively an immense number of interfaces between paper and air in 

 the cable, this effect must be of some importance. 



Increasing the temperature increases the thermal energy of the molecules 

 and their consequent thermal motion which helps maintain the random 

 orientation of the molecules. Thus, the thermal motion opposes the action 

 of the electric field in maintaining the alignment of the dipoles so that as 

 the temperature rises, the polarization is reduced. But in the cable there 

 are unequal expansions of the copper and the lead sheath which may act to 

 increase the internal pressure as the temperature rises, increasing dielectric 

 densities as well as bringing the wires closer together. 



The final result of all these effects on the capacitance of the cable pairs is 

 shown by the curves of Fig. 8, which give the 19-gauge capacitance-frequency 

 relations for several temperatures. Figure 9 shows the variation of 

 capacitance with temperature for several frequencies. The largest change 



