656 



Professor J. A. Fleming 



[June 4, 



It should be clearly understood that when a wu^eless telegraph 

 antenna is in operation, it sends out into the surrounding space a 

 nearly hemispherical electric wave which spreads out in all directions. 

 There are five causes which weaken the wave as it travels outwards : — 



1. The distribution of the energy continually over a larger and 

 larger area. The wave amplitude diminishes inversely as the distance, 

 and the wave energy inversely as the square of the distance. This 







Metres. 



■K= 



Specific Resistance in Ohms per Metre Cube. 



Fig. 2.— Depth op Penetration of Waves 1000 Ft. in Length. 

 (Dr. Zenneck.) 



is proved theoretically from first principles by Hertz's equations, and 

 has been confirmed experimentally by the experiments of Messrs. 

 Duddell and Taylor, and of Prof. Tissot. 



2. There is a certain absorption of energy due to the ionisation 

 of the atmosphere by dayhght and to other causes, 1)ut this is only 

 detectable over long distances, and for the present moment we shall 

 neglect it. We include, however, under this head, obstructions due 

 to special atmospheric conditions, electrical or material. 



?). There is a diminution due to earth curvature which is operative 

 only over long distances. 



4. There is some reduction of intensity wliich results from ob- 

 stacles — such as hills, trees — especially from cliffs of ironstone or 

 conductive rocks, due to distortion of the electric field. 



5. Lastly, there is the weakening due to the dissipation of energy 

 by the penetration of the waves into the surface over which they 

 travel. 



We shall consider the last-named cause alone at the present 

 moment. Dr. Zenneck has discussed mathematically, in a very 

 interesting paper, the effect of the conductivity and dielectric con- 

 stant of the terrestrial surface, soil or sea, on the propagation of a 

 plain electric wave over it, assuming the radiation to be from an 



