127.  All the effects of solution of metallic continuity, and the consequent diminution of power described by Messrs. Babbage and Herschel[A], now receive their natural explanation, as well also as the resumption of power when the cuts were filled up by metallic substances, which, though conductors of electricity, were themselves very deficient in the power of influencing magnets.  And new modes of cutting the plate may be devised, which shall almost entirely destroy its power.  Thus, if a copper plate (81.) be cut through at about a fifth or sixth of its diameter from the edge, so as to separate a ring from it, and this ring be again fastened on, but with a thickness of paper intervening (fig. 29.), and if Arago’s experiment be made with this compound plate so adjusted that the section shall continually travel opposite the pole, it is evident that the magnetic currents will be greatly interfered with, and the plate probably lose much of its effect[B].

  [A] Philosophical Transactions, 1825, p. 481.

  [B] This experiment has actually been made by Mr. Christie, with the
  results here described, and is recorded in the Philosophical
  Transactions for 1827, p. 82.

An elementary result of this kind was obtained by using two pieces of thick copper, shaped as in fig. 28.  When the two neighbouring edges were amalgamated and put together, and the arrangement passed between the poles of the magnet, in the direction parallel to these edges, a current was urged through the wires attached to the outer angles, and the galvanometer became strongly affected; but when a single film of paper was interposed, and the experiment repeated, no sensible effect could be produced.

128.  A section of this kind could not interfere much with the induction of magnetism, supposed to be of the nature ordinarily received by iron.

129.  The effect of rotation or deflection of the needle, which M. Arago obtained by ordinary magnets, M. Ampere succeeded in procuring by electro-magnets.  This is perfectly in harmony with the results relative to volta-electric and magneto-electric induction described in this paper.  And by using flat spirals of copper wire, through which electric currents were sent, in place of ordinary magnetic poles (Ill.), sometimes applying a single one to one side of the rotating plate, and sometimes two to opposite sides, I obtained the induced currents of electricity from the plate itself, and could lead them away to, and ascertain their existence by, the galvanometer.

130.  The cause which has now been assigned for the rotation in Arago’s experiment, namely, the production of electrical currents, seems abundantly sufficient in all cases where the metals, or perhaps even other conductors, are concerned; but with regard to such bodies as glass, resins, and, above all, gases, it seems impossible that currents of electricity, capable of producing these effects, should be generated in