DIELECTRIC PROPERTIES OF INSULATING MATERIALS 497 



constant interchangeably in the quaUtative discussion of the magnitude 

 of the dielectric polarization. In dealing with alternating currents 

 the fact that polarizations of the absorptive type require a time to 

 form which is often of the same order of magnitude as, or greater than, 

 the period of the alternations, results in the polarization not 

 being able to form completely before the direction of the field is 

 reversed. This causes the magnitude of the dielectric polarization 



ELECTRICAL 

 FREQUENCIES 



OPTICAL 

 FREQUENCIES 



Fig. 1 — Schematic diagram of variation of dielectric constant and dielectric 

 absorption with frequency for a material having electronic, atomic, dipole and 

 interfacial polarizations. 



and dielectric constant to decrease as the frequency of the applied 

 field increases. An example of this variation of the dielectric constant 

 with frequency is shown in the radio and power frequency section of 

 the curve plotted in Fig. 1. It is often convenient to refer to the mid- 

 point of the decreasing dielectric constant-frequency curve as the 

 relaxation-frequency; this frequency fm is very simply related to the 

 relaxation- time r, for the theory of these effects shows that/^ = l/27rr. 



