Prof. Thomson on the Mechanical Theory of Electrolysis, 439 



we find from the preceding expression, by substituting 20 X 5280 

 for «, be an electro-motive force of 1,549,000, or very nearly half 

 of that which is required for the decomposition of water. It 

 is not probable that the current produced by the action of this 

 force alone through the wire connecting the platinum plates 

 would be found to be sensible ; since a sensible continuous cur- 

 rent with water and platinum electrodes in the circuit can 

 scarcely be obtained by any electro-motive force less than that 

 which is required for the continued gaseous decomposition of 

 water. The existence of the inductive action might, however, I 

 think, be tested by using a galvanic battery of very low intensity, 

 to assist the electro-motive force arising from induction, and by 

 adding a little nitric acid to the liquid till it is found that a sen- 

 sible current is produced. It might then be observed whether or 

 not, when the tide turns and the water flows in the other direc- 

 tion through the channel, the electrical current becomes insensible, 

 or becomes less than it was ; and whether it goes on again as 

 before when the tide turns again, and the water flows as it did at 

 first. There would probably be some difficulty experienced in 

 keeping the electro-motive force of the battery sufficiently con- 

 . stant during twelve hours to make the experiment perfectly satis- 

 factory, and many difficulties that could not be foreseen might 

 occur. If, however, in any state of the tide the mean rate per 

 hour of the stream in the Channel exceeds two miles or two miles 

 and a half, it is probable that the inductive action might produce 

 a sensible electric current in the telegraph wire without such 

 assistance. It is very much to be desired that the experiment 

 should be tried, as it would afford probably the best test that could 

 at present be apphed, to find whether electrolysable liquids pos- 

 sess the property of magneto-electric induction discovered by 

 Faraday in metalHc conductors. 



17. The possible magneto-electric efiects of the eartVs rotation 

 were also considered by Faraday, and it was conjectured that 

 electricity may, in virtue of them, rise to considerable intensity. 

 The general nature of the effect was shown to be a tendency for 

 electricity to flow through the earth from the equator towards 

 the poles, from whence it would endeavour to return externally 

 to the equatorial regions. If the distribution of terrestrial mag- 

 netism were perfectly symmetrical about the axis of rotation, 

 there could be no other kind of effect than this produced by the 

 rotatory motion ; and, neglecting at present the currents in com- 

 plete external circuits, which may exist in virtue of the v/ant of 

 this symmetry, we may endeavour to form a rough estimate of 

 the electro-motive force that would exist between the equatorial 

 regions of the revolving mass and a quadrantal conductor fixed 

 relatively to the earth^s centre, with one end near the surface at 



