466 DR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 



described, I have thought it would be useful, in this investigation of the voltaic pile, 

 to notice them briefly here. 



1035. When the battery is in action, it causes such substances to be formed and 

 arrayed in contact with the plates as very much weaken its power, or even tend to 

 produce a counter current. They are considered by Sir Humphry Davy as sufficient 

 to account for the phenomena of Ritter's secondary piles, and also for the effects 

 observed by M. A. De la Rive with interposed platina plates*. 



1036. I have already referred to this consequence (1003 ), as capable, in some 

 cases, of lowering the force of the current to one eighth or one tenth of what it was 

 at the first moment, and have met with instances in which its interference was very 

 great. In an experiment in which one voltaic pair and one interposed platina plate 

 were used with dilute sulphuric acid in the cells (fig. 31.), the wires of communication 

 were so arranged, that the end of that marked 3 could be placed at pleasure upon 

 paper moistened in the solution of iodide of potassium at x, or directly upon the platina 

 plate there. If, after an interval during which the circuit had not been complete, the 

 wire 3 were placed upon the paper, there was evidence of a current, decomposition 

 ensued, and the galvanometer was affected. If the wire 3 were made to touch the 

 metal of p, a comparatively strong sudden current was produced, affecting the gal- 

 vanometer, but lasting only for a moment ; the eflfect at the galvanometer ceased, 

 and if the wire 3 were placed on the paper at x, no signs of decomposition occurred. 

 On raising the wire 3, and breaking the circuit altogether for a while, the apparatus 

 resumed its first power, requiring, however, from five to ten minutes for this purpose ; 

 and then, as before, on making contact between 3 and p, there was again a momen- 

 tary current, and immediately all the effects apparently ceased. 



1037. This effect I was ultimately able to refer to the state of the film of fluid in 

 contact with the zinc plate in cell i. The acid of that film is instantly neutralized by 

 the oxide formed ; the oxidation of the zinc cannot, of course, go on with the same 

 facility as before ; and the chemical action being thus interrupted, the voltaic action 

 diminishes with it. The time of the rest was required for the diffxision of the liquid, 

 and its replacement by other acid. From the serious influence of this cause in expe- 

 riments with single pairs of plates of different metals, in which I was at one time 

 engaged, and the extreme care required to avoid it, I cannot help feeling a strong 

 suspicion that it interferes more frequently and extensively than experimenters are 

 aware of, and therefore direct their attention to it. 



1038. In considering the effect in delicate experiments of this source of irregularity 

 of action in the voltaic apparatus, it must be remembered that it is only that very small 

 portion of matter which is directly in contact with the oxidizable metal which has to be 

 considered with reference to the change of its nature; and this portion is not very readily 

 displaced from its position upon the surface of the metal (582. 605.), especially if that 



* Philosophical Transactions, 1826, p. 413. 



