DIELECTRIC PROPERTIES OF INSULATING MATERIALS 505 



frequencies. The information obtainable from the study of such 

 residual currents is the same in kind as that obtainable from the study 

 of dielectric constant and conductivity by means of alternating cur- 

 rents; the residual phenomena, however, provide data regarding 

 polarizations having relaxation-times which are too long for convenient 

 investigation by alternating current methods. Residual currents of 

 this kind have no significance in principle which is different from that 

 of low-frequency a-c measurements. 



Conductivity and Dielectric Loss 



The conductivity of a material is usually thought of as a property 

 which depends upon the ease with which electric charge can be trans- 

 ferred through the material by the application of an electric field, 

 though it is recognized that a dissipation of electrical energy as heat 

 occurs in the material through which the current is passing. In these 

 terms we think of the conductivity as a quantity proportional to the 

 current per unit voltage gradient, which in turn is proportional to the 

 number of charge carriers, their mobility, and the magnitude of the 

 charge borne by each carrier. For conductors it does not matter 

 whether we define the conductivity, 7, as the factor by which the 

 voltage gradient, E, must be multiplied to give the current density, /, 



I = yE (1) 



or as the factor by which the square of the voltage gradient must be 

 multiplied to give the heat, W, developed per second in a unit cube of 

 the material,^ 



W = IE = yE\ (2) 



for the heat developed by a given voltage is proportional to the current, 



no matter of what material the conductor is composed. This is due to 



the fact that the energy obtained by the moving charges from the 



applied electric field is dissipated continuously to the surrounding 



molecules or lattice structure as heat, and the electrons or ions then 



drift with constant average velocity in the direction of the applied 



field, developing heat at a rate proportional to the current. 



However, the proportionality between current and heat developed 



which is characteristic of conductors does not obtain in dielectrics. 



When an alternating current flows in a dielectric it dissipates some 



electrical energy as heat; however, the amount is generally much 



smaller than would be dissipated by an equal current flowing in a 



^ Cf. for example, Mason and Weaver, "The Electromagnetic Field," Chicago 

 (1929), p. 233. 



