158 Electrical .Disturbances due to Tides and Waves. 



A marked increase is observable in the roughness of the zero 

 line under these conditions. The fluctuations in the E.M.F. 

 are frequently from 1 to I '5 millivolts indicating an 

 oscillating E.M.F. e 2 with an amplitude of from 05 to 0'75 

 millivolts. It should be explained that the galvanometer 

 was observed on open circuit for a short time in order to 

 ensure that the recorded deflexions were electrical and not 

 mechanical in origin. 



C. General Discussion of Experimental Results. 



In the experimental work recorded the examination of the 

 electrical effects of waves and tides was only incidental to 

 objects of more immediate practical importance. In a 

 systematic observation certain obvious improvements in the 

 methods are desirable. 



(a) Two moored electrodes will indicate the direction of 

 the E.M.F. recorded only to within 90° and will measure 

 only the component of the E.M.F. parallel to the line 

 between them. For complete measurements it is necessary 

 to have electrodes stationed on lines at right angles to one 

 another giving simultaneous records. 



(b) It is desirable that the experiments with moored and 

 towed electrodes should proceed simultaneously and in the 

 same area in order that the results may be correlated. 



(c) The experimental conditions would be simplified if an 

 area were chosen near a coast-line comparatively free from 

 indentations. Under such conditions refinements oE the 

 methods of measurement would probably prove profitable. 



The results obtained clearly indicate the existence of 

 periodic E.M.F/s in the sea whose period and magnitude 

 show them to be the result of earth-induction in tidal 

 currents. The high values obtained with drifting electrodes 

 indicate that the E.M.F/s due to purely local induction 

 which are measured by moored electrodes must be small, 

 and that the E.MJEVs actually measured by these electrodes 

 are due principally to more remote tidal currents ; this view 

 is supported by the phase differences observed. This merely 

 means that the tidal " dynamo" is almost completely shorted 

 by the earth return, a condition which might be expected. 

 The failure of Faraday to detect any tidal E.M.F. in the 

 Thames, in so far as it was not due to the disturbing con- 

 ditions mentioned by him, may have been due to such an 

 effect ; for assuming a nearly uniform velocity over the 

 cross-section of the stream, an earth return of low resistance 

 and an absence of currents generated at a distance, the 

 potential would be sensibly uniform across the stream. 



