658 



Professor J. A. Fleming 



[June 4, 



Dr. Zenneck has considered the case of electric waves 1000 feet 

 in wave length, and has represented the final result by some interest- 

 ing curves. He defines the effect of the absorption of energy by 

 the soil by stating the distance in kilometres at which the wave 

 amplitude would be reduced by the effect of this absorption to 

 0-367 = 1/c of its amplitude at the sending station, altogether apart 

 from the weakening due to the spreading of the waves out in a 

 hemisphere, which "we may call the spherical or space decrease. 

 These curves are plotted to abscissae representing the specific resist- 

 ance of the soil (see P'ig. 4). You will see from this diagram that 

 when a plane electric wave having the above wave-length is propagated 



XI. 



S.g> 10,000 



^-•i 1,000 

 •^ "fev 1 nn 



Si 



11 



II " 





80 



20 



I 

 10| 



5 5 



1 



Sea 

 Water. 



10 



Fresh 

 Water. 



100 1000 



Damp Soil. 



10,000 100,000 1,000,000 

 Dry Soil. Insulators. 



Specific Resistance in Ohms pea Metre Cube. 



Fig. 4. — Curves showing the Distance in which Electric Waves 

 1000 Ft. (300 Metres) in Length have Amplitude reduced to 

 1/e BY Travelling over various Surfaces. (Dr. Zenneck.) 



over sea water, it would have to travel 1(»,000 kilometres before its 

 amplitude would Ije reduced in the assigned ratio ; and over fairly dry 

 soil, about 100 to lUOO kilometres ; but over very dry soil, having a 

 small dielectric constant, only about 1 to 10 kilometres. Also you 

 will notice that the curves rise up again for still higher resistivities. 

 This, of course, is as it should be. All the practical cases lie between 

 two ideal extremes : the case of an infinitely perfect conducting earth, 

 in which case the waves would not penetrate into it at all : and the 

 other case, an infinitely perfect non-conducting earth, in which the 

 wave would penetrate into it, but would suffer no dissipation of 

 energy. This theory is quite in accordance with practical experience 



