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BELL SYSTEM TECHNICAL JOURNAL 



indicated schematically in Fig. 6. As is there shown, there must be an 

 initial stage, always too brief to be detected experimentally, during 

 which the inductance of the circuit and the inertia of the charges 

 cannot be neglected. 



The Types of Initial Conductivity 



The polarization conductivity 7poi may then be measured in two 

 ways: either as the infinite-frequency conductivity 7cc obtained by 

 a-c measurements, or as the initial conductvity 70 obtained by d-c 

 measurements : 



7pol = 7oo = 7o- 



The quantity 7poi refers to a property of the material, whereas 70 and y^, 

 refer to the methods of measuring this property. 



<z 



FREQUENCY 



' 2-rT 



CHARGING TIME, ic 



Fig. 6 — A-C and d-c methods of measuring conductivity. A schematic diagram 

 comparing the dependence of a-c conductivity (7') on frequency with the dependence 

 of apparent d-c conductivity 7c(/c) on charging time (/<;). For homogeneous dielec- 

 trics 70 = Too = 7poi, where Tpoi is a conductivity due to polar molecules or to bound 

 ions. For some non-homogeneous dielectrics 70 = 700 = 7/, where 7/ is a conductivity 

 due to free ions. 



Reference to Table I will show that there are several types of polari- 

 zation which may be responsible for 700. Consequently this will be 

 true also of the initial conductivity. The equality of 70 and 700 

 applies to all of these types of polarization and, therefore, can not 

 be used to distinguish between them. For this reason experimental 

 agreement between a-c and d-c measurements of conductivity does 

 not enable us to distinguish whether we are dealing with polar mole- 

 cules, bound ions, or free ions in a macroscopically inhomogeneous 

 dielectric. Thus it is clear that when we are dealing with a polariza- 

 tion due to polar molecules, the initial condtictivity is a true polarization 

 conductivity or a specific dielectric loss. If the polarization is of the 

 bound ion type discussed earlier in this paper, the initial conductivity 



