506 BELL SYSTEM TECHNICAL JOURNAL 



conductor and, unlike conductors, the ratio of heat developed to current 

 flowing varies with the material. This is due to the fact that most of 

 the current flowing in a dielectric under ordinary conditions is a 

 polarization current, or rather a sum of several polarization currents of 

 different types, and in general a polarization current dissipates less 

 energy as heat than an equal current flowing in a conductor. In fact 

 a part of the current flowing in a dielectric — the optical polarization 

 current — passes through the dielectric material without developing 

 any heat in it at the ordinary frequencies of electrical transmission. 

 Electrical energy can be transmitted through a good dielectric in a 

 suitable range of frequencies with very little loss; in other words, the 

 dielectric is transparent to currents which have a suitable frequency of 

 alternation. In these circumstances the conductivity of the material 

 as measured on a bridge would be very small though the current density 

 per unit voltage gradient might be quite large. Evidently, then, the 

 view of conductivity as simply a measure of the ease of transfer of 

 electric charge through a material is not in general suitable for ap- 

 plication to dielectrics. 



The fact is that the complex conductivity represents the ease of dis- 

 placement of electric charge in a dielectric while its real part (i.e., the 

 a-c conductivity as measured on a bridge or equivalent measuring 

 device) is the quantity to which the rate of heat development in the 

 material is proportional. Therefore, in dealing with alternating cur- 

 rents flowing in dielectrics it is usually more convenient to regard the 

 a-c conductivity as the factor which determines the rate of dissipation 

 of electrical energy as heat in the material, rather than as a quantity 

 w^hich is proportional to the current density per unit voltage gradient 

 or to the ease of displacement of electric charge in the material. In a 

 later part of the discussion, however, it will be shown that the limiting 

 high-frequency value of the a-c conductivity may be thought of as 

 representing ease of displacement of electric charge, too, as in a 

 conductor. 



The heat developed in a dielectric by polarization currents is called 

 dielectric loss and is analogous to the Joule heat developed by free 

 electrons or ions in a conductor; however, it is a property of neutral 

 aggregates of particles, such as polar molecules, rather than of free ions. 

 In the case of a polarization due to polar molecules, for example, the 

 equilibrium distribution of the orientations of the molecules is slightly 

 changed by the application of an electric field. The dielectric constant 

 depends upon the difference between the distribution of orientations 

 with and without the applied field, while the dielectric loss represents 

 the part of the energy of the applied field which is dissipated as heat 



