CHARACTERISTICS OF TOLL TELEPHONE CABLES 305 



the paper in cables varies from 1.7 to 1.9 depending upon the amount of air 

 and impurities contained in the paper. Of the space around the wires 

 inside the sheath about 40 per cent is filled with paper and the remaining 

 60 per cent with air. Frequency and temperature of the cable affect the 

 true dielectric constant in a complicated way. Slight amounts of moisture 

 remaining in the cable even after drying affect the dielectric constant and 

 the capacitance as well as the leakage conductance and introduce further 

 changes in the frequency-temperature characteristics. 



Results of an extensive study of the dielectric constant were given by 

 E. J. Murphy and S. 0. Morgan in a series of recent papers . They point 

 out (first paper, p. 494; second paper, p. 641) that a dielectric may be 

 thought of as an assemblage of bound charges, that is charged particles which 

 are so bound together that they are not able to drift from one electrode to 

 the other under the action of an applied electric field of uniform intensity. 

 But the applied field disturbs the equilibrium of the forces acting on the 

 bound charges and they take up new equilibrium positions, thereby increas- 

 ing their potential energy when the applied field is removed. Then when 

 the applied field is removed, some of this energy is dissipated as heat in 

 the dielectric. If the applied field is alternating, the bound charges swing 

 back and forth with certain amplitudes and the sum of the product of the 

 amplitude by the charge extended over all the bound charges in a unit 

 volume determines the dielectric constant of the material. The energy 

 dissipated as heat by the motions of the bound charges is the dielectric loss, 

 which is proportional to the a-c. conductivity after the d-c. conductivity has 

 been subtracted from it. 



Considering the fact that positive and negative charges will be displaced 

 in opposite directions and such a motion constitutes an electric current, there 

 is thus what is called a polarization current or charging current flowing while 

 the polarization (or displacement of charges) is being formed. K the 

 current alternates too rapidly for the polarization to form completely before 

 the field reverses its direction, the magnitude of the dielectric polarization 

 and the dielectric constant will be reduced. The result of this lag, therefore, 

 is that the dielectric constant (and likewise the capacity) decreases with 

 increasing frequency. This is the phenomenon known as anomalous dis- 

 persion from its relation to the anomalous dispersion of light, i.e., at visible 

 frequencies. 



A further important concept in dielectric theory is that the molecules of 

 all dielectrics except those in which the positive and negative charges are 

 symmetrically located, possess a permanent electric moment characteristic 



'1 E. J. Murphy and S. O. Morgan, "The Dielectric Properties of Insulating Materials," 

 B.S.T.J. XVI (1937) pp. 493-512; XVII (1938) pp. 640-669; XVIII (1939) pp. 502-537. 

 These are referred to as "First Paper," etc. 



