WAVEGUIDE TRANSMISSION 305 



It therefore corresponds to a flow of power. In the notation just referred to, 

 it may be expressed by the vector equation 



P = E X H (6.1-4) 



4. Lines of force exhibit the properties of inertia. They therefore resist ac- 

 celeration. 



Other principles not quite so fundamental but nevertheless useful in 

 application are : 



5. Lines of force are under tension and at the same time are under lateral 

 pressure. 



6. For perfect conductors there can be no tangential component of electric 

 force. That is to say, lines of electric force when attaching themselves to a 

 perfect conductor must approach perpendicularly. This is substantially 

 true also for common metals such as copper. 



In passing it is well to point out that the first principle is really that by 

 which the ordinary dynamo operates. The second is, for practical purposes. 

 Oersted's Principle, if we assume that the Unes of electric force are attached 

 to charges flowing in near-by conductors. The third is known as the Poynting 

 Principle. It has a wide field of application contributing very materially to 

 the physical pictures of both radio and waveguide transmission. When ap- 

 plied to the very simple case of low frequencies propagated along a trans- 

 mission line, it gives a result that is in keeping with the usual view that the 

 power transmitted is equal to the product of the total voltage times the total 

 current. The fourth principle is useful in explaining qualitatively how radia- 

 tion from an antenna takes place. The usefulness of these four principles will 

 be made more evident by the examples that follow. 



6.2 Transmission of Power along a Wire Line 



Direct Current 



According to the Poynting concept, one may think of an ordinary dry 

 cell as two conductors combined with chemical means for producing a con- 

 tinuous supply of lines of electric force. This need not be counter to the ac- 

 cepted views concerning electrolysis, for we may think of these lines of force 

 as being attached to ionic charges incidental to dissociation. As long as the 

 cell is on open circuit, these lines of electric force remain in a static condition 

 in which many are grouped in the neighborhood of the terminals of the cell 

 as shown in Fig. 6.2-1 (a). In this state of equilibrium, the forces of lateral 

 pressure are balanced by the forces of tension. There is no motion and hence 

 no flow of power. For an ordinary dry cell such as used in flashlights, the 

 electric intensity E will depend on the spacing of electrodes, but it may be 

 as much as 200 volts per meter If we attach to the dry cell two parallel 

 wires spaced perhaps a centimeter apart with their remote ends open, electro- 



