Prof. PoggendorfF on the Zinc-Iron Circuit. 43 



Now the electromotive force between zinc and iron is, it is 

 true, smaller than that between zinc and copper, silver or pla- 

 tina, which is evident from this, that when it is combined in the 

 opposite sense with one of the last-mentioned circuits into a 

 system, it is immediately overpowered by it: but that resist- 

 ance in two circuits which are alike in all except in the nature 

 of the plates, constitutes the sole or essentially different ele- 

 ment; viz. the resistance of transition is, as Fechner has shown, 

 in general likewise small in metals which are attacked by the 

 liquid of the circuit. If now, as might hence be supposed, this 

 resistance is smaller in iron with acids and solutions of salts 

 than in copper, and indeed in still greater proportion smaller 

 than the electromotive force between zinc and iron, in com- 

 parison to that of zinc and copper, it is evident that, all other 

 circumstances being equal, the current of the zinc-iron circuit 

 must be stronger than that of the zinc-copper circuit. 



If, however, the iron circuit owes its greater intensity of 

 current to the smallness of its resistance of transition, then its 

 current must possess a slighter tension than that of the cop- 

 per circuit, or in other words, it must be weakened by the in- 

 sertion of a foreign resistance of some consideration in a greater 

 proportion than that of the latter circuit. The testing of this 

 circumstance must decide as to the correctness of this expla- 

 nation. 



The author has undertaken such an examination, it is true, 

 in want of an apparatus of measurement, only with the help 

 of a common galvanometer, which for quantitative determina- 

 tions is an ill-suited instrument, but will however afford for the 

 present case a sufficient approximation. The presupposition 

 was made that the intensity of the current must be propor- 

 tional to the tangent of the deviation. This presupposition is 

 evidently false: the force increases in a greater proportion 

 than the tangent of the angle of deviation, since the needle on 

 rotation "oes further from the convolutions of the wire. But 

 exactly because the force increases in greater proportion than 

 the tangent of the angle of deflection, — the stronger forces, 

 therefore, were estimated in a greater proportion too small 

 than the weaker,— the conclusions drawn from the measure- 

 ments under that presupposition must deserve the more con- 

 fidence. 



The plates employed were all of equal size, 1 inch broad, dip- 

 ped 2-5 inches into dilute sulphuric acid, and stood 5 lines from 

 each other. The current of the two circuits, that of zinc-iron 

 and that of zinc-copper, were successively measured in the 

 above-mentioned manner under two circumstances; once when 

 the circuit was merely closed by the ll-foot long and ^-line 



