ANALYSIS OF THE IONOSPHERE 



479 



A magnetic field, the earth's or any other, should have no effect 

 whatsoever on radio waves so long as these are traveling in air com- 

 posed entirely of neutral molecules. When, however, the waves are 

 setting electrons into motion, the moving electrons are affected by 

 the field, which has a twisting action on their paths. We have seen 

 already that the moving electrons react, so to speak, upon the waves, 

 raising the wave-speed thereof. By altering the motions of the 

 electrons, the magnetic field will influence at second hand the waves 

 themselves. But will the result be perceptible? In view of the fact 



600 - 



uj 400 



5-.00 5:20 5:40 6:00 



6:20 6:40 7:00 

 TIME OF DAY 



7:20 7:40 8:00 8:20 8:40 



Fig. 12 — Ceiling for signal of fixed frequency shifting near sunrise from F to E as 

 the electron-density of the £-Iayer increases with increase of light. (Appleton,) 



that the earth's magnetic field is very feeble by comparison with the 

 fields between the poles of our electromagnets great or small, or even 

 with those around the horseshoe magnets which are playthings, one 

 might well think the influence not worth the trouble of computing. 

 But those who first undertook to compute it — Nichols and Schelleng 

 in America, Appleton independently in England, in the winter of 

 1924-25 — found it a serious influence, and very well worth the trouble. 

 The problem is one of those which are not very hard to state, but 

 can be very tedious to solve except in special cases which may or may 

 not be of practical importance. For this problem it happens that 

 two of the special cases can be solved with relative ease, and one at 



