L 1« J 



XIII. On Electrical Disturbances due to Tide* and Waves. 

 Bi/ F. B. Young, B.A., D.Sc, H. Gerhard, iU.Sc, and 

 W. Jevons, M.Sc.* 



TPlate II.] 

 Introduction. 



IT was pointed out by Faraday f that when a stream of 

 water flows through the earth's magnetic field an E.M.F. 

 should be induced in the water. He carried out some ex- 

 periments at Waterloo Bridge, lowering metallic electrodes 

 from the parapet into the tide-way at various distances apart 

 up to a maximum of 960 feet. Deflexions were obtained 

 but were very irregular and were in succession referred to 

 other causes than that sought for, such as difference of con- 

 centration and of temperature, slight differences in the 

 plates, etc. 



In the course of certain sea experiments which were 

 carried out during the year 1918, electrical disturbances 

 were encountered which were definitely traced to movements 

 of masses of sea- water in the earth's magnetic field. It was 

 found in fact that the sea is a region of considerable electro- 

 magnetic activity. The observations in question are com- 

 municated with the permission o£ the Admiralty. 



I. Theoretical. 



A. E.M.F. induced in Stream of Water. 



A stream of water may be regarded as composed of a 

 collection of transverse filaments which in their motion cut 

 through the lines of the earth's vertical magnetic field. An 

 E.M.F. is therefore induced along each of these filaments 

 from right to left as one faces down-stream. Assume the 

 velocity to be uniform throughout the cross section of 

 the stream. If e is the E.M.F., v the velocity of the stream, 

 s the length of the filament, and V the earth's vertical field, 

 then 



e = Vvs electromagnetic units or Yvs x 10 " 8 volt. 



If V is taken as 0'43, and v as 50 cm. /sec. (i. e. about 

 1 knot), then <? = 2'15 x 10~ 7 volt per cm. per knot. 



* Communicated by F. E. Smith, O.B.E., F.R.S. 

 t Bakerian Lecture, Roy. Soc. 12th January, 1832. 



