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XIX. Theory of Voltaic Action. By J. Brown, Esq.* 



THE production of a difference of electric potential by vol- 

 taic action is attributed by some primarily to the differ- 

 ence of chemical attraction between the two elements of a 

 voltaic couple for one of the components (ions) of some com- 

 pound body (electrolyte) in contact with both, that element 

 which has the greater affinity being the positive one. It is 

 said by others to be due to the simple " contact " of the two 

 elements without the intervention of any third substance or 

 combination, and has been attributed, in the case of two metals 

 such as copper and zinc, to their mutual chemical attraction "f. 

 Faraday could not, however, discover any current during the 

 combination of two metals (tin and platinum), through great 

 heat was evolved % ; and he considered that though the source 

 of energy in a voltaic pair was the combination of the active 

 ion with the positive plate, decomposition was necessary to its 

 development in the form of electricity. Numerous old expe- 

 riments may be cited which show how in* various ways altera- 

 tions in the electric relations of metals in voltaic pairs may be 

 produced without altering their contact. 



The following experiments seem to go far towards establish- 

 ing the truth of the first-mentioned (chemical) theory. 



If a potential series (A) be formed by immersing couples of 

 various metals &c. in an oxidizing electrolyte and testing for 

 the current generated, and another (B) by the use of condenser- 

 plates in the usual way adopted by contact theorists, the two 

 series will be found curiously similar. The simplest conclusion 

 appears to be that the so-called " contact" excitement is due 

 to the presence of a gaseous film § containing water, carbon 

 dioxide, or other oxygen compounds between the plates, which 

 film may be considered as having all the properties of an oxi- 

 dizing electrolyte except its conductivity. 



If in forming series A we use an electrolyte containing 

 some other active ion such as sulphur, we obtain a totally dif- 

 ferent series, which, as Professor Fleeming Jenkin remarks ||, 

 is " quite anomalous and inconsistent with the simple poten- 

 tial theory." But if the chemical theory be true, then in 

 forming series B, if we substitute for the ordinary atmosphere, 

 containing watery vapour and other oxygen compounds, an 



* Communicated by the Author. 



t Sir William Thomson, ' Electrostatics and Magnetism,' § 400 ; Tait, 

 1 Kecent Advances/ p. 805 et seq. 

 X Phil. Trans. 1834, p. 436. 

 § Wiedemann, Galvanismus, p. 12. 

 !| Electricity and Magnetism, p. 217. 



