304 BELL SYSTEM TECHNICAL JOURNAL 



The quantity )u is the magnetic permeability of the medium under considera- 

 tion. 



2. Lines oj electric force, ivhen displaced lalerally, induce in the immediately 

 adjacent space lines of magnetic force. The direction of the induced magnetic 

 force is perpendicular to the direction of motion and also perpendicular to the 

 direction of the original electric force. The intensity H of the induced magnetic 

 force is proportional to the velocity v of displacement and proportional to the 

 intensity E of the original lines of electric force. 



UNIT VOLUME CONTAINING 

 STORED ENERGY 



POYNTING VECTOR 

 FLOW OF POWER 



Z 



Fig. 6.1-5. Directions of the vectors E and H relative to the Poyntlng vector P in an 



advancing wave front. 



Again Fig. 6.1-3 and also the right-hand or cork-screw rule apply. In the 

 short-hand notation these facts may be expressed by the following vector 

 equation: 



H = €(vxE) (6.1-2) 



In this equation, e is the dielectric constant of the medium.- 



3. When an electric field of intensity E is translated laterally, it together with 

 its associated magnetic field H represents a flow of energy. The direction of the 

 flow of energy is perpendicular to both E and H and is therefore in the direction 

 of the velocity v. The magnitude of the energy flow per unit volume across a unit 

 area measured perpendicular to v is proportional to the product of the electric 

 intensity E and the magnetic intensity H. // may be designated by the vector P. 

 The relative directions of the vectors P, E, and H are shown in Fig. 6.1-5. 

 The energy per unit volume moves with a velocity expressed by 



V = —r^ (6.1-3) 



Viue 



* The values of permeability ju and dielectric constant e appearing in these equations 

 are not the values found in most tables of the properties of materials. As here given p. is 

 smaller than the usual value ^r by a factor of 1.257 X 10 "^ while e is smaller than tr by 

 a factor of 8.854 X 10"'^. The use of these special values leads to certain mathematical 

 simplifications. 



