187.1.] Polarization of Metallic Surfaces in Aqueous Solutions. 243 



III. " Polarization of Metallic Surfaces in Aqueous Solutions, a new 

 Meth od of obtaining Electricity from Mechanical Force, and certain 

 relations between Electrostatic Induction and the Decomposition 

 of Water." By Cromwell Fleetwood Varley. Communicated 

 by Prof. Sir W. Thomson, F.R.S. Received October 5, 1870. 

 (Abstract.) 



Platinum plates immersed in sulphuric acid and water, as in a decompo- 

 sition-cell, require a potential of about 1*7 Daniell's cell to decompose the 

 water ; with potentials of less amount the platinum plates can be charged 

 and discharged like condensers. They have enormous electrostatic capa- 

 city. Mercurial surfaces equally admit of polarization with hydrogen. A 

 surface of mercury in dilute sulphuric acid, when made negative to the water 

 by means of a powerful battery, flattens out. If the mercury be replaced 

 by an amalgam of proper consistency, the flattening out is increased ; the re- 

 versal of the current restores the amalgam to its original dimensions. By 

 reversing the process, electric currents can be obtained from mechanical force. 



A large vessel on a board has within it two shallow funnels, which are 

 connected by means of glass tubes with similar vessels outside of the large 

 one. Pure mercury is poured into the funnels until they and the outside 

 vessels are one-third filled. By tilting the board, mercury runs into the 

 one funnel and out of the other, and thus the surface in the one is made 

 to increase while that in the other decreases. Dilute sulphuric acid is 

 poured into the larger vessel so as to cover the two funnels ; the latter 

 are connected together through a galvanometer. 



If the mercury be pure and free from polarization, the tilting of the 

 board produces no electric current. On polarizing one of the surfaces with 

 hydrogen by a battery, it gives rise to a current through the galvanometer, 

 and thus shares the polarization over the two surfaces. If the battel y be 

 removed, on augmenting the one surface and diminishing the other, a cur- 

 rent of electricity is seen to pass through the galvanometer. 



A convenient method of showing this experiment on a large scale is to 

 procure a gutta-percha trough 4 inches deep and 4 by 2 inches broad. A 

 partition of the same material 2 inches high divides the lower half into two 

 separate chambers : these are partly filled with mercury ; amalgamated 

 platinum plates, hung from a balance-lever, dip into the mercury. On 

 depressing one set of plates the others are elevated, and thus the mercurial 

 surface exposed to the fluid is alternately augmented and diminished to a 

 large amount. Twelve of these arranged in series give a current of rather 

 more potential than one cell of Daniell's battery when the mercury is polar- 

 ized with hydrogen. The addition of a minute fragment of zinc to the 

 mercury maintains the polarization for a very long time, and the power is 

 considerably increased thereby. When a large surface of mercury (25 cir- 

 cular inches) has been polarized with a power of half a Daniell's cell and is 

 rapidly reduced to the diameter of fa inch, by letting the mercury flow out 



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