DIELECTRIC PROPERTIES OF INSULATING MATERIALS 643 



Pi is a unit vector in the direction of the vector (si — S2). If now one 

 of these charges is an electron {e = 4.77 X 10"^" e.s.u.) and the other 

 a unit positive charge and they are separated by a distance of the order 

 of magnitude of atomic distances (10~^ cm.), p has the value 4.77 

 X 10~^^ e.s.u., or 4.77 Debye units. The permanent electric moments 

 of molecules seldom exceed a few Debye units. 



Let us now apply the definition contained in equation (1) to a 

 dielectric material. In the first place it indicates that if we know the 

 effective positions of the electrons and other charged particles which 



Fig. 1 — The calculation of the polarization vector by the general method for a very 



simple configuration. 



contribute to the structure of the material we can always, in principle, 

 calculate the polarization of the body as a whole or any part of it. 

 Actually the calculation of the polarization of a body as a whole or 

 that of unit volume in it is in general a complicated matter involving 

 statistical considerations, but there are special cases in which the result 

 is rather obvious. For example, in a gas or liquid if all orientations of 

 the molecules are equally probable in the absence of an applied field, 

 the value obtained by taking the time-average of the summation indi- 

 cated by (1) is zero. Equation (1) would also give the value zero when 

 applied to all of the ions in a c.c. of a solution because any arbitrarily 

 chosen small volume in the liquid would be as likely to contain a posi- 

 tive ion as a negative ion. 



