494 BELL SYSTEM TECHNICAL JOURNAL 



and absorption curves for light and those for electromagnetic dis- 

 turbances in the electrical (i.e., radio and power) range of frequencies. 

 The difficulty is then met that the quantum-mechanical model is 

 the customary medium of description of the absorption of light. 

 But, since the references to optical properties will be only incidental 

 and for comparative purposes, there is little to be lost, even in this 

 domain in which quantum-mechanical concepts are the familiar 

 medium of description, in using the pre-quantum theory concepts of 

 dispersion and absorption processes. Thus a model operating on the 

 basis of classical mechanics and the older conceptions of atomic struc- 

 ture will be sufficient for our present purposes. 



On the wave-mechanical theory of the structure of matter a di- 

 electric is a material which is so constructed that the lower bands of 

 allowed energy levels are completely full at the absolute zero of temper- 

 ature (on the Exclusion Principle) and at the same time isolated from 

 higher unoccupied bands by a large zone of forbidden energy levels.^ 

 Thus conduction in the lower, fully occupied bands is impossible 

 because there are no unoccupied energy levels to take care of the 

 additional energy which would be acquired by the electrons from the 

 applied field, while the zone of forbidden energy levels is so wide that 

 there is only a negligible probability that an electron in the lower band 

 of allowed levels will acquire enough energy to make the transition to 

 the unoccupied upper band where it could take part in conduction. 

 The bound electrons in a completely filled and isolated band of allowed 

 levels can, however, interact with the applied electric field by means of 

 the slight modifications which the applied field makes in the potential 

 structure of the material and hence in the allowed levels. 



On the other hand in the older theory of the structure of matter the 

 essential condition which makes a material a dielectric is that the 

 electrons and other charged particles of which it is composed are held 

 in equilibrium positions by constitutive forces characteristic of the 

 structure of the material. When an electric field is applied these 

 charges are displaced, but revert to their original equilibrium positions 

 when the field is removed. In this account of the behavior of di- 

 electrics this model will be sufficient, but no essential change in the 

 relationships which will be discussed here would result if a translation 

 were made to a model based upon quantum-mechanics. 



When an electric field is impressed upon a dielectric the positive 



and negative charges in its atoms and molecules are displaced in 



opposite directions. The dielectric is then said to be in a polarized 



' Cf., for example, Gurney, "Elementary Quantum Mechanics," Cambridge 

 (1934); (Herzfeld, "The Present Theory of Electrical Conduction," Electrical Engi- 

 neering, April 1934. 



