210.  The copper was then compared in a similar manner with tin, lead, and iron, and surpassed them all, even more than it did zinc.  The zinc was then compared experimentally with the tin, lead, and iron, and found to produce a more powerful current than any of them.  Iron in the same manner proved superior to tin and lead.  Tin came next, and lead the last.

211.  Thus the order of these metals is copper, zinc, iron, tin, and lead.  It is exactly their order with respect to conducting power for electricity, and, with the exception of iron, is the order presented by the magneto-rotation experiments of Messrs. Babbage, Herschel, Harris, &c.  The iron has additional power in the latter kind of experiments, because of its ordinary magnetic relations, and its place relative to magneto-electric action of the kind now under investigation cannot be ascertained by such trials.  In the manner above described it may be correctly ascertained[A].

[A] Mr. Christie, who being appointed reporter upon this paper, had it in his hands before it was complete, felt the difficulty (202.); and to satisfy his mind, made experiments upon iron and copper with the large magnet(44.), and came to the same conclusions as I have arrived at.  The two sets of experiments were perfectly independent of each other, neither of us being aware of the other’s proceedings.

212.  It must still be observed that in these experiments the whole effect between different metals is not obtained; for of the thirty-four feet of wire included in each circuit, eighteen feet are copper in both, being the wire of the galvanometer coils; and as the whole circuit is concerned in the resulting force of the current, tin’s circumstance must tend to diminish the difference which would appear between the metals if the circuits were of the same substances throughout.  In the present case the difference obtained is probably not more than a half of that which would be given if the whole of each circuit were of one metal.

213.  These results tend to prove that the currents produced by magneto-electric induction in bodies is proportional to their conducting power.  That they are exactly proportional to and altogether dependent upon the conducting power, is, I think, proved by the perfect neutrality displayed when two metals or other substances, as acid, water, &c. &c. (201. 186.), are opposed to each other in their action.  The feeble current which tends to be produced in the worse conductor, has its transmission favoured in the better conductor, and the stronger current which tends to form in the latter has its intensity diminished by the obstruction of the former; and the forces of generation and obstruction are so perfectly neutralize each other exactly.  Now as the obstruction is inversely as the balanced as to conducting power, the tendency to generate a current must be directly as that power to produce this perfect equilibrium.