1909] 



on Researches in Radiotelegraphy . 



655 



lators. Dry sand or hard dry rocks are poor conductors, but wet sand 

 and moist earth are fau'lj good conductors. Sea water, owing: to the 

 salt in it, is a much better conductor than fresh water. The follow- 

 ing table gives some figures, which however are only approximate, for 

 the specific resistance of various terrestrial materials in ohms per 

 metre cube. It will be seen that dry sand or soils are of very high 

 specific resistance, and damp or wet sand or clay fairly low. 



Table I. — Approximate Conductivity and Dielectric Constant 

 OP VARIOUS Terrestrial Materials. 



If our earth's surface had a conductivity equal say to that of 

 copper, then the electric radiation from an antenna would glide over 

 the surface Avithout penetration. In the case of the actual earth there 

 is, however, considerable penetration of the wave into the surface, and 

 therefore absorption of energy by it. 



Brylinski and also Zenneck have calculated the depth to which 

 electric Avaves of such frequency as are used in radiotelegraphy pene- 

 trate into the sea or terrestrial strata of various conductivities. For 

 mathematical reasons, it is customary to define it by stating the 

 depth in metres or centimetres at which the wave amplitude is 

 reduced to 1/e = 0*867 of its amplitude at the surface. I have 

 represented in a diagram some of Zenneck's results calculated for 

 waves of 1000 feet in length, and for terrestrial surface materials of 

 various kinds, conductivities and dielectric constants (see Fig. 2). 

 You will see that in the case of sea water an electric wave travelling 

 over it penetrates only to the depth of a metre or two, whereas in 

 the case of very dry soil it would penetrate much deeper. Owing to 

 the conductivity of the soil, this movement of lines of magnetic force 

 tlu'ough it, sets up currents of electricity which expend their energy in 

 heat. This energy must come from the original store imparted to 

 the sending antenna, and therefore the wave is robbed of its energy as 

 it travels over the surface. 



