488 Prof. Thomson on the Mechanical Themy of Electrolysis, 



it may be very vague, of the anticipated effects. Thus let us 

 conceive a mass of air or water, or any other substance moving 

 relatively to the earth with a velocity V, and let A and B be 

 two fixed points in it or at its two sides, at a distance a apart, 

 in a line perpendicular to the direction of motion. Then if F 

 be the component of the terrestiial magnetic force perpendicular 

 to the plane of AB and the lines of motion across it, there will 

 be between A and B, or between any fixed conductors connected 

 with them, and insulated in all other places from the moving 

 mass, an electro-motive force, the intensity of which is given by 

 the equation 



2 = F.V.a. 



15. If, for instance, the velocity be one mile per hour, we 

 should have V= 1*4667; and if we take F = 10, which will be 

 nearly the case for a mass moving horizontally in any part of 

 Great Britain*, we have 



«= 14-667 x«. 



If we take fl! = 960, we find 2 = 14080 for the electro-motive 

 force between two platinum plates immersed, as in Faraday's 

 experiment, below the surface of the Thames, at a distance of 

 960 feet apart across the stream, when the tide is in such a state 

 that the current is at the rate of a mile an hour. The electro- 

 motive force, varying directly as the rate of the current, must 

 therefore, when there is a current of two miles and a half an hour, 



be 35200, which is very little more than y^ of that which 



was found in § 13 for the intensity required to decompose water ; 

 and as there is probably in no state of the tide a current of more 

 than three or four miles an hour, it is not to be wondered at 

 that no galvanic current was discovered in a wire connecting the 

 platinum plates. 



16. An experiment on a much larger scale might be performed 

 by means of the telegraph wires which have recently been laid 

 between England and France, across the straits of Dover, by 

 simply connecting the ends of one of these wires with platinum 

 plates immersed in the sea on the two sides of the channel. If 

 the distance between the plates be twenty miles, in a direction 

 on the whole at right angles to the direction of the motion of the 

 water through the channel, and if, in a particular state of the 

 tide, there be an average velocity of a mile an hour, there would, as 



* In June 1846 the horizontal magnetic force was found to be 37284, 

 and the dip 68° 58', at Woolwich (Philosophical Transactions, 1846, p. 246). 

 Hence the vertical force was 37284 X tan 68° 68', or 9-696. At the same 



Eeriod it was 994 at Manchester, and it must have been 10 exactly at loca- 

 ties in England or Scotland not far north of Manchester. 



