Propagation of Electric Waves Over the Earth 



By H. W. NICHOLS and J. C. SCHELLENG 



S\ Mii-.|s: Thf lomiMMlivi-ly |K)or lr,iiismis.sii)n of r.iilin waves of Iwo or 

 ihri'f luindrt'il iiifttTS iiulicatcs soino sort of selective elTect in the alinos- 

 pluTe. Siith .111 etTcel is foiiiul to result from the cxisterue of free electrons • 

 ni the .itniosphere when the niaKnetir field of the earth is taken into airount. 

 In the earth's niasnetie tielil, which is alioilt one-half gauss, this selective 

 etiect will ixcnr .it a w.ive leUKth of approxintalelv 200 meters. Ionized 

 hydrogen nioletules or atoms result in resonant effects at frequencies of 

 a few hundritl cycles, this Ix'ing outside of the radio range. The pa|K'r, 

 however, takes into account the eflfects of ionized molecules as well as 

 ekitrons. 



The result of this coml)ination is that the electric vector of a wave travel- 

 ing ikirallel to the magnetic field is rotated. Waves traveling (wrpendicular 

 to the magnetic field undergo doul)le refraction. Critical elTccts are 

 observeil in rotation, bending of the wave and absorption at the resonant 

 frequency. The paper develops the mathematical theory of these phe- 

 nomena and gives formulas for the various etTccts to be expected. 



THE problem of the propagation over tlie earth of electromagnetic 

 waves such as are used in radio communication has attracted 

 the attention of a niimher of investigators who have attacked the 

 prnlilein along somewhat different lines, with the purpose of offering 

 an explanation of how electromagnetic wa\'es can affect instrimienis 

 at a great distance from the source in spite of the curvature of the 

 earth. N'o attempt will be made here to describe adequately the 

 various theories, but we remark that the theories of diffraction around 

 a conducting sphere in otherwise empty space did not give satisfactor>- 

 results and led to the necessity for the invention of a hypothetical 

 conducting layer (Heaviside layer) whose aid is invoked to confine 

 the wave between two concentric spherical shells. In many cases 

 this Heaviside layer was considered to have the properties of a good 

 conductor and it was supposed that a beam of .short waves, for ex- 

 ■iinple, might be more or less regularly reflected back to the earth. 

 The high conductivity of this layer was supposed to be due to the 

 ionizing action of the sun f>r of particles invading the earth's atmos- 

 |)here from outside and producing in the rarefied upper atmosphere a 

 high degree of ionization. The differences in transmission during 

 day and night and the \'ariations which occur at sunrise and sunset 

 were supposed to \)C due to the different ionizing effects of the sun's 

 rays appropriate to the different times of day. The explanation of 

 the phenoment)n of "fading" or comparatively rapid fluctuations in 

 the intensity of received signals could then be built up on the assump- 

 tion of irregularities in the Heaviside layer producing either inter- 

 ference between waves arriving by different paths or reflcrtion to 

 different points on the earth's surface. The principal difiticully in 



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