On the Polarization of Platinum Plates. 4*7 



Now 



S2 _ r 2 \/l+ tan 2 fo _ r A _ 

 ^i r\ s/ 1 -Ttan 2 ^ r t ' 



that is, if r 2 is considerably lower than r u E 2 will be in the 

 same proportion lower than E l7 and hence R may be a good 

 deal smaller than the impedance of the primary. 



LX. On the Polarization of Platinum Plates. 

 By 0. H. Dkaper/_S.A, B.Se* 



r pHE fact of the decomposition of dilute sulphuric acid by 

 J- the passage between platinum electrodes of electricity 

 through a voltameter containing it, involves an expenditure 

 of energy. This energy is made available by a sudden fall 

 of the current through a certain difference of potential nume- 

 rically equal to the energy absorbed by the quantity of water 

 which one unit of electricity decomposes. The cause of this 

 sudden fall of potential within the voltameter over and above 

 that due to the resistance of the liquid as a conductor is the 

 modified condition assumed by the platinum plates, which 

 leads to the phenomenon called polarization. Those portions 

 of the gaseous products which come first into contact with 

 the platinum, especially in the case of hydrogen, either form 

 with the platinum a chemical combination, or undergo such 

 physical or chemical modification by occlusion or condensation 

 as results in a loss of energy and resulting fall of potential. 

 This modification of condition progressively diminishes in 

 the successive layers as we proceed outwards from the pla- 

 tinum plate, until a layer is reached which is beyond the 

 reach of the influence of the platinum, and where the gas 

 escapes freely as fast as it is formed. The result of this 

 polarized condition of the electrodes is manifested as an 

 electromotive force opposed to that which produces the cur- 

 rent. The electromotive force of polarization has for each 

 electrolyte a theoretically fixed maximum value, and expe- 

 rimentally there appears to be an approximately constant 

 maximum value which is always above the theoretical value 

 when any considerable current is passing, and which varies 

 with the conditions of the experiment. Thus Professor Tait, 

 in some experiments described in the ' Philosophical Maga- 

 zine ' for September 1869, found (taking the electromotive 



* Communicated by the Author. 



2K '1 



