RADIOTELEGRAPHY—FLEMING. 163 
We shall consider the last-named cause alone at the present moment. 
Doctor Zenneck has discussed mathematically, in a very interesting 
paper, the effect of the conductivity and dielectric constant of the 
terrestrial surface, soil, or sea, on the propagation of a plain electric 
wave over it, assuming the radiation to be from an ordinary vertical 
antenna, and the electric force therefore normal to the earth and 
magnetic force parallel to it. The result is to show that there are, 
broadly speaking, three cases to consider. First, supposing the sur- 
face 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 alternating force parallel 
to it, and there is very little subterranean electric or magnetic force 
(see fig. 3a). This is realized approximately 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 dielec- 
tric constant; then analy- 
sis shows that the elec- 
tric force in the air has 
two components, one per- 
pendicular to the earth’s 
surface and one parallel 
to it, and the resultant is 
an alternating and a ro- 
tating force, the direc- = +=10hm ” = 100,000 ohms + = 10,000 ohms 
D : 20 per metre cube. per metre cube. per metre cube. 
tion of its maximum K = 80. K=2 -3. K=1—3. 
value beins inclined to ry = Resistivity. K = Dielectric constant. 
5 F 
Fie. 3. 
the surface and leaning 
forward (see fig. 38). The wave front therefore slopes forward. 
Also, there is a subterranean electric force, showing that the wave 
is penetrating into the soil, and there is therefore dissipation of 
energy owing to the conductivity of the soil as the wave travels over 
the surface. This case is realized 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 con- 
stant from 2 to 3 and a specific resistance of about 10,000 ohms per 
meter cube—for example, very dry earth or sand. Then the investi- 
gation shows that the electric force in the air has two components, 
one parallel to the earth’s surface and one perpendicular to it differ- 
ing 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. 8c). At the 
same time there is some penetration of the wave into the earth and 
consequent dissipation of energy. 
