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BELL SYSTEM TECHNICAL JOURNAL 



In some crystalline materials equation (1) gives the value zero be- 

 cause there is a suitable symmetry in the configuration of charged 

 particles in the unit cell ; for other solids equation (1) gives a finite value 

 for the unit cell, but zero when applied to a volume of the material 

 large enough to contain a great many crystallites with random orienta- 

 tions; however, there are some macroscopic crystals which have per- 

 manent polarizations. A solid material consisting of polar crystallites 

 with random orientations is analogous, as far as equation (1) is con- 

 cerned, to a liquid or gas containing polar molecules having random 

 orientations; the polarization of the material as a whole is zero in 

 either case. 



CONDENSER- 

 PLATES 



• POSITIVE CHARGE 

 O NEGATIVE CHARGE 



Fig. 2 — A dielectric in a condenser. The circles joined by a bar represent "bound 

 charges" of various kinds, including atoms and molecules. 



Let us now consider a dielectric of any kind occupying the space 

 between two plane, parallel condenser plates of great enough area and 

 small enough separation that the electric field between the plates when 

 they are charged may be considered to be directed normally to them 

 (cf. Fig. 2). Consider the space between the plates of the condenser 

 to be divided into small cubes of the same size, the purpose of this 

 imaginary division of the dielectric being merely to obtain a representa- 

 tive specimen of the dielectric material. If the cube size is too small 

 the instantaneous value of p obtained by applying equation (1) to all 

 of the particles in a cube will vary appreciably from one cube to another ; 



