PROPAGATIOS Ol- l-.I.ECTRIC If.iriiS OfLR TUE EARTH 227 



!•%• the iii.i^;nclic fivM .uul will lia\e the curvature t';. The two com- 

 (xtnents into whioli tiie original \va\e is resf)lve(i will travel with 

 different \elocities. It is clear that when the distrihution of ions 

 in the up|H'r atmosphere is changed by varying sunlight conditions, 

 the resulting effect at a receiver is likely to \ary considerably. Sorae 

 of the possibilities will In.' discussed later. 



Rotation of the plane of polarization. It has been shown that in 

 the second case, namely transmission along the magnetic fiekl, there 

 will Ih; a rotation of the plane of polarization of the wave. This 

 rotation is such that the wave is rotated through a ii)ni()lfit' iiirn in a 

 distance given by 



" Mo <jN or' ^ ' 



«o- ur — \ 



It is interesting to note that the distance in which a long wa\e rotates 



through 2 V approaches the constant value — ^ as the wave length 



(jN 



increases and that for very short waves the rotation of the plane of 

 polarization tends to vanish with the wave length. 



Absorption. When an electron strikes a massive neutral atom the 

 a\erage persistence of velocities is negligible and in the steady state 

 of motion of electrons and neutral molecules the element of convection 

 current represented by an impinging electron will be neutralized, so 

 far as the wave is concerned, at every collision. Of the energy which 

 has been put into this element of convection current since the last 

 collision, a part will l)e spent in accelerating neutral molecules, part 

 will go to increase the average random velocity of the electron and a 

 part will appear as disordered electromagnetic radiation. Thus, as 

 far as the wave is concerned, the process of collision with massive 

 neutral molecules is irreversible even if the molecules are elastic, 

 and all the energy picked up by the electron from the wave between 

 collisions is taken from the wave at the next collision. Exactly the 

 same state of affairs would exist if at each collision the electron recom- 

 binerl with a molecule and a new electron were created with zero or 

 random velocity. Thus for massive molecules for which we can 

 neglect the persistence of electron velocities the effect upon the wave 

 is exactly the same whether the collision is elastic or inelastic. 



These conclusions are verified by the results of two different com- 

 putations which we have made of the resistance term, re, in equation 



