480 BELL SYSTEM TECHNICAL JOURNAL 



least is realizable in practice. I cannot venture to give the theory 

 of even this one, but at least I will attempt to describe what happens. 



The special case occurs when the waves are traveling at right angles 

 to the field. Since they travel vertically,^ it is necessary to find a 

 place on the earth where the field is horizontal. Such places are found 

 in the equatorial regions only, and these are not precisely crowded 

 with universities or engineering experiment stations. However, the 

 Carnegie Institution of Washington was inspired, several years ago, 

 to set up a station in just such a place: Huancayo, in the Andes of 

 Peru. Here they established long straight horizontal antennae, one 

 running north-and-south, another east-and-west, and yet another 

 northeast-and-southwest. In the waves which mount from these to 

 the ionosphere and then come bouncing back, the electric field Eq sin nt 

 — henceforth to be called "the electric vector" — is faithful to the 

 direction of the antenna. They are called "plane-polarized waves." 



When the north-south antenna is used, the electrons are impelled 

 to and fro in the north-south direction which is that of the magnetic 

 meridian. Now as is well known, an electron moving parallel to a 

 magnetic field behaves just as it would if there were no such field at 

 all. These waves ought therefore to behave according to the theory 

 which we set up while we were still disregarding the magnetic field. 

 They are the so-called "ordinary waves" or "o-waves." 



When the east- west antenna is used, the electrons of the ionosphere 

 are impelled to and fro in the east-west direction, which is transverse 

 to the earth's magnetic field. This is just the condition for the 

 maximum amount of meddling by the field in the motion of the 

 electrons. The meddling consists in bending the electron-paths into 

 curiously twisted arcs. The action of the magnetic field i? tantamount 

 to strengthening the electron-current-density le parallel to the electric 

 vector. It will be recalled (from page 464) that it is /« which for 

 small iV-values cancels a part of the displacement-current and so 

 speeds up the waves, and for a certain critical iV-value cancels the 

 whole of the displacement-current and so brings about total reflection. 

 So, for these "extraordinary waves" or ":x;-waves," a given iV-value 

 produces a greater augmentation of the wave-speed, and the critical 

 A/'-value for total reflection is smaller, than for the ordinary waves. 

 The signal composed of x-waves, mounting into the ionosphere, finds 

 its appropriate mirror at a lesser altitude than does the signal composed 

 of o-waves, and it gets earlier back to earth. It may indeed come 



* In addition to the other advantages of sending the waves up vertically, there 

 is the feature that the angle between their line of motion and the field is the same 

 when they are going up and when they are returning. 



