DIELECTRIC PROPERTIES OF INSULATING MATERIALS 495 



condition, and since the motion of charges of opposite sign in opposite 

 directions constitutes an electric current there is what is called a 

 polarization current or charging current flowing while the polarized 

 condition is being formed. 



For the case of a static impressed field a charging current flows in 

 the dielectric only for a certain time after application of the field, the 

 time required for the dielectric to reach a fully polarized condition. 

 If the material is not an ideal dielectric, but contains some free ions, 

 the current due to a static impressed field does not fall to zero but to 

 a constant value determined by the conductivity due to free ions. 

 More important than the static is the alternating current case, where 

 the potential is continually varying and where, consequently, there 

 must be a continuously varying current. 



The dielectric behavior of different materials under different con- 

 ditions is reflected in the characteristics of the charging or polariza- 

 tion currents, but since polarization currents depend upon the applied 

 voltage and the dimensions of condensers it is inconvenient to use 

 them directly for the specification of the properties of materials. 

 Eliminating the dependence upon voltage by dividing the charge by 

 the voltage, we have the capacity (C = Q/V); and the dependence 

 upon dimensions may be eliminated by using the dielectric constant, 

 defined as e = C/Co, where C is the capacity of the condenser when the 

 dielectric material is between its plates and Co is the capacity of the 

 same arrangement of plates in a vacuum. The dielectric constant 

 is then a property of the dielectric material itself. 



The term "dielectric polarization" is used to refer to the polarized 

 condition created in a dielectric by an applied field of either constant 

 or varying intensity. The polarizability is one of the quantitative 

 measures of the dielectric polarization; it is defined as the electric 

 moment per unit volume induced by an applied field of unit efi"ective 

 intensity. Another quantitative measure of the dielectric polarization 

 is the molar polarization; this is a quantity which is a measure of the 

 polarizability of the individual molecule, whatever the state of ag- 

 gregation of the material. 



The concept of polarizability is as fundamental to, and plays about 

 the same role in, the theory of dielectric behavior as does the concept 

 of free ions in the theory of electrolytic conduction. Just as the con- 

 ductivity of a material is a measure of the product of the number of 

 ions per unit cube and their average velocity in the direction of a unit 

 applied field, so the polarizability is a measure of the number of bound 

 charged particles per unit cube and their average displacement in the 

 direction of the applied field. 



