on the Intensity of Current of the Zinc-iron Circuit. 107 



herent power of resistance to the reception of electric fluid, 

 existing in a greater degree in copper than in iron ; and if I 

 understand him rightly, he supposes this to be not a less 

 facility of conduction in the copper, but a resistance to the 

 entrance of the electric fluid into the substance of the metal 

 from another conducting medium in contact with it; that is 

 to say, it is more difficult for electricity to enter copper from 

 an acidulated solution in which this metal is immersed, than 

 to enter iron in like circumstances, although the copper is 

 actually a better conductor than iron when the electric fluid 

 has once penetrated its substance. This I grant to be the fact, 

 but not that the cause is any such singular and contradictory 

 property in copper as first to offer a greater and then a less 

 resistance to the passage of electricity ; but it is simply that 

 copper, when immersed in an acidulated solution, does not 

 retain so clean a metallic surface as does iron when exposed 

 to a like action. 



When a copper-zinc pair is placed in dilute sulphuric acid 

 an action takes place upon both the metals, and the balance 

 of their affinities for the acid determines the direction and 

 intensity of the electric current ; but an obstacle to its free 

 circulation arises by the resistance offered to its passage from 

 the acid into the copper, because this metal has in a measure 

 been acted upon by the acid and its surface partially oxidated ; 

 but as the affinity of the base for the acid under these circum- 

 stances is not sufficient to cause the solution of the oxide, 

 it therefore remains upon the surface of the copper plate, and 

 as oxides are worse conductors of electricity than their metal- 

 lic bases, we have here a resistance presented by the oxidated 

 surface to the entrance of the electric current into the copper 

 plate. On the other hand, when an iron-zinc pair is immer- 

 sed in dilute acid, we have also an action on both metals, but 

 the balance of affinities is here not so much in favour of the 

 zinc as when it is in combination with copper, therefore the 

 intensity or electromotive force generated by the iron-zinc is 

 not so great as in the copper-zinc battery; but as I have 

 shown, the quantity circulated by the iron-zinc is greater, be- 

 cause the surface of the iron not only oxidates as did the cop- 

 per, but in consequence of its greater affinity for the acid this 

 oxide becomes dissolved in the liquid, and it is thus removed 

 from the surface of the metal, which remains purely metallic, 

 bright, and far more fitted to conduct electricity than would 

 be the oxidated surface of a copper plate; it therefore offers 

 less resistance to its entrance, and a larger quantity is thus 

 circulated, although (in consequence of the balance of affini- 

 ties) in less intensity or electromotive force by an iron-zinc 

 than by a copper-zinc galvanic pair. 



