194 AKlSniAL REPORT SMITHSONIAN INSTITUTION,, 1942 



electrical conductivity as a source of magnetic and auroral effects 

 observed on the earth. Even over 100 years ago Gauss, the outstand- 

 ing pioneer in advancing scientific consideration of the earth's 

 magnetic field, referred to this possibility in his earlier writings. 

 Subsequently, such scholars as Schuster, Kennelly, Heaviside, Lo- 

 rentz, Eccles, and Lamor took part in the development of the concept. 



The highly electrified regions outside the earth were again sug- 

 gested in 1901 in order to explain Marconi's success in sending radio 

 signals across the Atlantic Ocean from England to Newfoundland. 

 Prior to Marconi's proof of the usefulness of radio signals for long- 

 distance communication, the public, including the scientists, were 

 inclined to ridicule his invention as a new gadget that would be limited 

 to communication without wires over line-of-sight distances only. 

 Was it not true that these Hertzian waves traveled in straight lines 

 like light? They would go over the horizon and be lost in space. 

 Hence, it would not be possible to communicate from one point to 

 another over the curved surface of the earth. Both Kennelly and 

 Heaviside independently suggested that a conducting region in the 

 outer atmosphere would be capable of bending radio waves around 

 and returning them back to earth. Radio waves travel with the 

 speed of light, 300,000 kilometers per second. If the frequency or 

 number per second is low, the distance between crests, or the wave 

 length, is long; for example, the wave length for 1,000 kilocycles or 

 one megacycle per second is 300 meters. If the frequency is high, the 

 wave length is short ; for example, the wave length for 15,000 kilocycles 

 or 15 megacycles per second is 20 meters. The short waves vibrate 

 more rapidly and have greater penetrating power. 



It was not until 1925 that Applcton in England and Breit and Tuve 

 in America, using radio waves by somewhat different methods, suc- 

 ceeded in directly "seeing" the radio-reflecting region of the outer 

 atmosphere. The echo-method technique developed by Breit and Tuve 

 for this purpose has now been adopted almost universally. By this 

 method a short pulse of radio waves is sent upward and the time meas- 

 ured for the echo to return. An equivalent height to which the wave 

 would travel can be computed, assuming it to have traveled at the 

 velocity of light. The density of ionization which reflects the wave 

 at this level is measured by the wave frequency transmitted. Corre- 

 sponding to each wave frequency is a particular density of ions which 

 will reflect the wave. 



There has been developed by the Department of Terrestrial Mag- 

 netism of the Carnegie Institution of Washington and the National 

 Bureau of Standards from the earlier experimental equipment a so- 

 called ionospheric apparatus which produces records automatically by 

 photographing echoes of vertically directed variable-frequency radio 



