1909] 



on Researches in Radiotelegraphy. 



657 



ordinary vertical antenna, and the electric force therefore normal to 

 the earth, and magnetic force parallel to it. The resnlt is to show 

 that there are, broadly speaking, three cases to consider. First, 

 supposing the surface material to be a good conductor, then the wave 

 moves over the surface and penetrates a very little way into it. The 

 electric force in the air over the surface is a purely alternating force 

 vertical to the earth's surface, and the magnetic force is an alter- 

 nating force parallel to it, and there is very little sul)terranean 

 electric or magnetic force (see Fig. :->, a). This is realised approxi- 

 mately or most nearly in the case of radiotelegraphy over sea water. 

 Secondly, let the earth be assumed to have a very poor conductivity 

 and not a very large dielectric constant, then analysis shows that the 

 electric force in the air has two components, one perpendicular to the 

 earth's surface and one parallel to it, and the resultant is an alter- 

 nating and a rotating force, the direction of its maximum value 

 being inclined to the surface and leaning forward (see Fig. 3, b). 



Air 



Sea 



A B C 



r = 1 ohm r = 100,000 ohms r = 10,000 ohms 



per metre cube. per metre cube. per metre cube. 



K = 80. K = 2 - 3. K = 1 - 3. 



r =■ Resistivity. K = Dielectric constant. 



Fig. 3. 



The wave-front therefore slopes forward. Also there is a subter- 

 ranean electric force, showing that the wave is penetrating into the 

 soil, and there is therefore dissipation of energy owing to the con- 

 ductivity of the soil as the wave travels over the surface. This case 

 is realised when the wave travels over land composed of dry soil 

 having a small dielectric constant. Thirdly, let the earth be a very 

 poor conductor, having a small dielectric constant from 2 to 3, and 

 a specific resistance of about 10,000 ohms per metre cube. For 

 example, very dry earth or sand. Then the investigation shows that 

 the electric force in the air has two components, one parallel to the 

 earth's surface and one perpendicular to it differing in phase, and 

 the resultant is represented by the rotating radius of an ellipse, the 

 maximum value or major axis of which is inclined forward in the 

 direction of the wave motion (see Fig. 3, c). At the same time there 

 is some penetration of the wave into the earth and consequent 

 dissipation of energy. 



