On the Conductivity of Sea-water, 89 



tbem cannot generally be said, though the researches of 

 Sir J. J. Thomson on the conductivity o£ electrolytes under 

 very rapidly alternating currents of frequencies up to 10 8 * 

 would lead one to expect the conductivity of sea-water 

 to be constant within the range of frequencies between 

 and 10 6 . 



Professor J. A. Fleming and Mr. G. B. Dyke, on the 

 other hand f, found the conductivity of various materials as 

 glass, celluloid, paraffin-wax, mica, paper, slate, and sulphur 

 to be a function of the frequency and increasing with the latter, 

 so for instance they found the conductivity of ebonite under 

 4600 cycles per second 6*4 times greater than under 920 

 cycles. For gutta-percha under a frequency as low as 800 a 

 conductivity was obtained already 100,000 times greater than 

 the value usually given for direct currents in the handbooks, 

 while at higher frequencies this ratio still increased J. 

 Professor Fleming therefore suggested to me to determine 

 the conductivity of sea-water under frequencies as used in 

 wireless telegraphy, and to compare it with the value found 

 under steady or slowly alternating currents. 



A simple calculation shows, assuming the conductivity of 

 sea-water has its normal value cr — -5.10 -11 , that up to the 

 frequency 10 6 the dielectric displacement current in sea- 

 water (the dielectric constant being assumed e=81) can be 

 neglected in comparison witli the conduction current, the 



ratio of the two being ~ — ^ where _p is the angular frequency 

 and c the velocity of light. For the material under consi- 

 deration this ratio amounts to -. , -,^'n n , ^ A = 1*4. 10~ 4 : 



47T.5.10 - u .9.10^° ' 



so that in order to measure the conductivity of sea-water a 

 method can be applied in which the latter is treated as only 

 having a resistivity. 



To use a bridge method as employed by Fleming and 

 Dyke (modified Wien bridge) did not seem advisable for 

 high frequencies, as serious errors introduced by the inductive 

 effects of the arms had to be avoided. The following sub- 

 stitution method appeared to be very reliable and exact (see 

 fig. 1). 



* J. J. Thomson, Roy. Soc. Proc. vol. xlv. p. 269 (1889). 

 t Journal Inst. Electr. Eng. London, vol. xlix. p. 323 (1912). 

 X See paper cited. 



