160 ANNUAL REPORT SMITHSONIAN INSTITUTION,, 1942 



the compass needle from its normal magnetic position. We may infer, 

 therefore, that at times of sunspot maxima the number of these ions in 

 the upper air is materially increased, producing a more marked mag- 

 netic effect. The strength of the magnetic field of the earth, therefore, 

 may be considered as increasing and decreasing with the variation in 

 the intensity of the ionization of the upper air that changes with sun- 

 spot occurrences. Most of our knowledge of the ionized region has 

 come about through the invention of the radio. 



In the early days of wireless, it was thought that electric waves 

 which carried telegraph messages without wires traveled in straight 

 lines over the earth, just as light waves do. With this conception 

 one could never hope to communicate over very great distances, since 

 the curvature of the earth would prevent the passage of the waves as 

 the earth's huge bulk bulged into the communication path. The earlier 

 wireless engineers thought that only by building higher and higher 

 antenna towers could one ultimately hope to communicate over the 

 thousands of miles that would make transoceanic wireless possible. 



Of course, these early crude notions about the way in which electric 

 waves travel were erroneous. Such, nevertheless, is the way in which 

 science has groped into the unknown. Somebody experimenting with 

 wireless and listening in found himself quite unconsciously eaves- 

 dropping on Marconi waves from the other side of the Atlantic. 

 Instantly the thought about how wireless waves travel had to be 

 changed. Evidently the electromagnetic waves followed the curvature 

 of the earth and did not travel in straight lines after all. This led 

 Professor Kennelly of Harvard to postulate that there must exist high 

 above the earth's surface, perhaps 100 miles or so up, an electrified con- 

 ducting layer from which the electromagnetic waves emitted from the 

 powerful antennae were reflected back to earth. The earth's upper 

 atmosphere, therefore, in his mind formed a conducting layer and 

 imprisoned the radio waves between the earth's surface and space 

 outside. A few months after Professor Kennelly published his 

 hypothesis, the English scientist, Oliver Heaviside, announced a simi- 

 lar conclusion quite independently. In honor of these two distin- 

 guished men this upper region of the earth's atmosphere that is 

 electrically ionized is commonly referred to as the Kennelly-Heaviside 

 layer, also designated as the E layer. 



If we look at a diagram (pi. 2) which presents a vertical section of 

 the earth and its atmosphere, we see that this Kennelly-Heaviside layer 

 exists at an altitude of from 100 to 130 kilometers. Radio waves 

 emitted from a sending station in all directions arriving in this ionized 

 region have their velocity and direction changed as they penetrate 

 farther and farther into the region, until at length they are bent 

 back to earth again, reaching receiving stations hundreds and some- 

 times thousands of miles from the source whence they were broadcast. 



