[ 88 ] 



,ur- 



VI. On the Value of the Conductivity of Sea-water for Ci 

 rents of Frequencies as used in Wireless Telegraphy. By 

 Balth. van der Pol, Jun., JDocts. Sc. (Utrecht)*. 



I^HE materials of the earth's crust, over and through 

 which the electromagnetic waves sent out by a wireless 

 telegraphy station travel, have an important influence on the 

 variations of the wave amplitude with distance from the 

 sending station. This fact was first found experimentally, 

 but was afterwards confirmed by some theoretical in- 

 vestigations f • 



When a plane bounding surface is assumed to exist between 

 the air and the earth, the magnitudes of the conductivity of 

 most materials of which the earth's crust consists are such, 

 that for the range of wave-lengths used in wireless telegraphy 

 and for the dielectric constants these materials possess, apart 

 from the divergence diminution, another decrease of wave 

 amplitude with distance, due to absorption, can in general be 

 expected. This latter diminution is principally determined 

 by the conductivity of the materials over which the waves 

 travel. 



When, on the other hand, the above-mentioned boundary 

 surface, in closer approximation to the actual circumstances, 

 is supposed to be a sphere, consisting of sea-water, and if the 

 value of the conductivity of the latter as found under direct 

 or slowly alternating currents is used in the calculations!, it 

 appears that a greater wave amplitude for the same sender 

 can be expected than would be obtained if the sphere were 

 made up of an infinitely good conducting material, though 

 the difference is small. 



The greater part of wireless traffic being conducted over 

 sea, an exact determination of the value of the conductivity 

 of sea-water for alternating currents of high frequencies 

 may be of importance, especially in connexion with the 

 divergence between the decrease of wave amplitude with 

 distance predicted by theory and values of the latter found 

 experimentally. 



That the conductivity of all materials is independent 

 between wide limits of the frequency of the currents in 



* Communicated by Sir J. J. Thomson. 



t Zenneck, Ann. d. Phys. Bd. xxiii. p. 846 (1907). Soinmerfeld, Ann. 

 d. Phys. Bd. xxviii. p. 665 (1909). 



X Macdonald, Proc. Roy. Soc. (Ser. A), vol. xcii. p. 493 (1916). 

 Love, Roy. Soc. Phil. Trans. (Ser. A) vol. ccxv. p. 105 (1915). See also 

 a paper shortly to be published in the Proc. Roy. Soc. by G. N. Watson. 



