398 



According to the observations of Weber, Joule, and others, the 

 quantity of water decomposed by a current of unit strength during 

 the unit of time, that is, by the electro -magnetic unit of electricity, 

 is very exactly -g^th of a grain. Hence from 2,000,000 to 8,200,000 

 electro-statical units are required to decompose a grain of water. A 

 positive and a negative electro-statical unit at a foot distance attract 

 one another with a force of ^ of the weight of a grain. Hence if 

 the electricities separated in the decomposition of a grain be concen- 

 trated in two points a foot asunder, they will attract with a force of 

 more than 10 tons, and less than 42 tons ! Faraday long ago conjec- 

 tured that less electricity passes in the greatest flash of lightning than 

 in the decomposition of a drop of water, which is now I think ren- 

 dered very probable. 



The expression for the force, in British dynamic units, between 

 two plates, each of area S, at a small distance, a, asunder, when 

 connected with the two poles of a battery of which the electro- 



S / F \ 2 * 

 motive force in electro- magnetic units is F, is ( ) , or in terms 



Sir \ oaj 

 1 ^ / F 1 \2 



of the weight of a grain . f ] . If F be the electromotive 



32'2 OTT \ffa/ 



force of 100 cells of Daniell's, which, as I have found from Joule's 

 observations, must be about 250,000,000f, and if a be -jV-h f a foot, 

 and S a square foot, I conclude from the preceding estimates for a, 

 that the force of attraction between the plates cannot be less than 

 4*4 grains, nor more than 72 grains. 



It would be easy at any time to make a plan for observing tele- 

 graph indications by means of either Weber's electro-dynamometer, 

 or an instrument constructed on the same principle, or by mea- 

 suring thermal effects of intermittent currents, which could be 

 put in practice by any one somewhat accustomed to make observa- 

 tions, and which would give a tolerably accurate determination of 

 the element of time, even in cases where the observable retarda- 

 tion is considerably less than ^th of a second. A single wire in a 

 submarine cable would, as far as regards the physical deductions to be 

 made from this determination, be to be preferred to one of a number 

 of different wires insulated from one another under the same sheath- 



* As was shown at the conclusion of a paper " On Transient Electric Currents," 

 published in June 1853, in the Philosophical Magazine, 

 t See the paper referred to above, as published in the Phil. Mag., Dec. 1851. 



