Electromagnetic Waves round the Earth. 



373 



order of magnitude as the electric force. It only happens 

 that at 6 = 7T the interference zones are such that H^ has 

 its minimum there and equals zero, while Er just attains a 

 maximum. In the accompanying diagram (fig. 1) we give 



Fig. 1. 



10 I/O 



antipodes 



an illustration of the amplitude of the theoretical value near 

 9 — 7T of the electric force for a wave of length 10 kilometres 

 travelling over a sphere of 40,000 kilometres circumference. 

 The ordinates of the curves max. and min. denote on an 

 arbitrary scale the maximum and minimum amplitude of the 

 electric force, while the abscissse give distances from the 

 antipodes measured in wave-lengths. Between these two 

 curves the actual electric force oscillates, forming inter- 

 ference zones with about ^X distance between them. Very 

 near the antipodes the distance between two successive zones 

 is a little bigger than \ X in accordance with the successive 

 zero's of l x (yd'). In order to show how these interference 

 zones are formed a few have been drawn in the small inset 

 at a larger scale. It is further seen that an increase of wave- 

 amplitude is to be found near 6 = 77, but it does not attain 

 such a value that the point 6 = tt can be considered to be 

 a focus in the optical sense. Moreover, the figure shows 

 clearly the gradual transition of nearly pure interference 

 with regions of zero-intensity to the zones farther removed 

 from the point = tt, where the interference becomes less 

 pronounced till it finally reaches a stage where only single 

 waves are present and where the effect of the disturbance 

 that travelled over the arc (tir — 6) is negligible. That 



