498 BELL SYSTEM TECHNICAL JOURNAL 



Various types of polarization can be induced in dielectrics: There 

 should be an electronic polarization due to the displacement of electrons 

 with respect to the positive nuclei within the atom; an atomic polari- 

 zation due to the displacement of atoms with respect to each other in 

 the molecule and in certain ionic crystals, such as rock salt, to the 

 displacement of the lattice ions of one sign with respect to those of the 

 opposite sign; dipole polarizations due to the effect of the applied 

 field on the orientations of molecules with permanent dipole moments; 

 and finally interfacial (or ionic) polarizations caused by the accumula- 

 tion of free ions at the interfaces between materials having different 

 conductivities and dielectric constants. 



Electronic Polarizations 



A classification of dielectric polarizations into rapidly-forming or 

 instantaneous polarizations and slowly-forming or absorptive polariza- 

 tions has been made. Instantaneous polarizations may be thought of 

 as polarizations which can form completely in times less than say 10~^° 

 seconds, that is, at frequencies greater than 10'" cycles per second or 

 wave-lengths of less than 1 centimeter, and so beyond the range of 

 conventional dielectric constant measurements. The electronic polari- 

 zations are due to the displacement of charges within the atoms, and 

 are the most important of the instantaneous polarizations. The 

 polarizability per unit volume due to electronic polarizations may be 

 considered to be a quantity which is proportional to the number of 

 bound electrons in a unit volume and inversely proportional to the 

 forces binding them to the nuclei of the atoms. 



The effect of number of electrons and binding force is illustrated by 

 a comparison of the values for the polarizability per unit volume of 

 different gases; for the number of molecules per unit volume is inde- 

 pendent of the composition of the gas. Thus, although a c.c. of 

 hydrogen with two electrons per molecule has the same number of 

 electrons as a c.c. of helium, which is an atomic gas with two electrons 

 per atom, the quantity e — 1, that is the amount by which the di- 

 electric constant is greater than that of a vacuum, is nearly four times 

 as large for hydrogen as for helium. This shows that in hydrogen the 

 electrons are in effect less tightly bound to the nucleus than in helium, 

 resulting in a larger induced polarization. Nitrogen has a larger 

 dielectric constant than either hydrogen or helium because it has 14 

 electrons per molecule. Some of these are tightly bound as in helium 

 and some are more loosely bound as in hydrogen. 



The dielectric constant of liquid nitrogen is 1.43, which is much 

 higher than the value 1.000600 for the gas. This is due to the fact 



