150 Messrs. Young, Grerrard, and Jevons on Electrical 



This is equivalent to about 2 millivolts per hundred yards 

 per knot. 



If the banks and bed of the channel are non-conducting 

 no current will be produced. If the banks are conducting, 

 an electric current will flow from right to left across the 

 stream, returning through the earth. The current density C 

 will reach a maximum when the resistance of the earth 

 return circuit is negligible in comparison with that of the 

 water ; in this case 



Cm»x=V»/pE.M. units, 



where p is the specific resistance of the water. 



B. E.M.F. acting hetween Two Stationary Electrodes. 



Let two electrodes A and B be stationed on a line trans- 

 verse to the stream at a distance 5 apart and let them be 

 connected by stationary leads through a voltmeter G (PI. II. 

 fig, 1). Then the E.M.F. e x acting from A to B through 

 the voltmeter will depend upon (a) the value of e ; (b) the 

 current density C. Its value will be given by 



e l = — e-{-Cps. 



If the resistance of the earth return is infinite so that C = 0, 

 then e x = — e or Yvs. 



If the resistance of the earth return is negligible so that 

 = Vi?//5, then ei = 0. 



Thus ei may have any value between and — Yvs. 



C. E.M.F. acting between r lwo Drifting Electrodes. 



If the electrodes and their connexions are allowed to drift 

 with the tide (as is the case if the former are suspended 

 from a drifting vessel), then an E.M.F. equal to e is induced 

 in the connecting cable in just the same manner as in the 

 water filaments. Thus if <? 2 is the E.M.F. measured by the 

 voltmeter, 



e 2 = ei-{-e = Gps. 



e { and e 2 are opposite in sign and are together numerically 

 equal to e. Thus as the resistance of the banks diminishes, 

 the value of e 2 will rise to a maximum value of Yvs. 



D. Effects of non-uniform Velocity and Conducting Bed. 



The conditions assumed in B and C unfortunately never 

 exist in nature, so that any precise verification of the theory 



