135.  Two rough trials were made with the intention of constructing magneto-electric machines.  In one, a ring one inch and a half broad and twelve inches external diameter, cut from a thick copper plate, was mounted so as to revolve between the poles of the magnet and represent a plate similar to those formerly used (101.), but of interminable length; the inner and outer edges were amalgamated, and the conductors applied one to each edge, at the place of the magnetic poles.  The current of electricity evolved did not appear by the galvanometer to be stronger, if so strong, as that from the circular plate (88.).

136.  In the other, small thick discs of copper or other metal, half an inch in diameter, were revolved rapidly near to the poles, but with the axis of rotation out of the polar axis; the electricity evolved was collected by conductors applied as before to the edges (86.).  Currents were procured, but of strength much inferior to that produced by the circular plate.

137.  The latter experiment is analogous to those made by Mr. Barlow with a rotating iron shell, subject to the influence of the earth[A].  The effects obtained by him have been referred by Messrs. Babbage and Herschel to the same cause as that considered as influential in Arago’s experiment[B]; but it would be interesting to know how far the electric current which might be produced in the experiment would account for the deflexion of the needle.  The mere inversion of a copper wire six or seven times near the poles of the magnet, and isochronously with the vibrations of the galvanometer needle connected with it, was sufficient to make the needle vibrate through an arc of 60 deg. or 70 deg..  The rotation of a copper shell would perhaps decide the point, and might even throw light upon the more permanent, though somewhat analogous effects obtained by Mr. Christie.

  [A] Philosophical Transactions, 1825. p. 317.

  [B] Ibid. 1825. p. 485.

138.  The remark which has already been made respecting iron (66.), and the independence of the ordinary magnetical phenomena of that substance and the phenomena now described of magneto-electric induction in that and other metals, was fully confirmed by many results of the kind detailed in this section.  When an iron plate similar to the copper one formerly described (101.) was passed between the magnetic poles, it gave a current of electricity like the copper plate, but decidedly of less power; and in the experiments upon the induction of electric currents (9.), no difference in the kind of action between iron and other metals could be perceived.  The power therefore of an iron plate to drag a magnet after it, or to intercept magnetic action, should be carefully distinguished from the similar power of such metals as silver, copper, &c. &c., inasmuch as in the iron by far the greater part of the effect is due to what may be called ordinary magnetic action.  There can be no doubt that the cause assigned by Messrs. Babbage and Herschel in explication of Arago’s phenomena is the true one, when iron is the metal used.