248.  Then, with regard to the production of electricity in these cases:—­whenever motion was communicated by the plate to the magnets, currents existed; when it was not communicated, they ceased.  A marked pole of a large bar magnet was put under the edge of the plate; collectors (86.) applied at the axis and edge of the plate as on former occasions (fig. 38.), and these connected with the galvanometer; when the plate was revolved, abundance of electricity passed to the instrument.  The unmarked pole of a similar magnet was then put over the place of the former pole, so that contrary poles were above and below; on revolving the plate, the electricity was more powerful than before.  The latter magnet was then turned end for end, so that marked poles were both above and below the plate, and then, upon revolving it, scarcely any electricity was procured.  By adjusting the distance of the poles so as to correspond with their relative force, they at last were brought so perfectly to neutralize each other’s inductive action upon the plate, that no electricity could be obtained with the most rapid motion.

249.  I now proceeded to compare the effect of similar and dissimilar poles upon iron and copper, adopting for the purpose Mr. Sturgeon’s very useful form of Arago’s experiment.  This consists in a circular plate of metal supported in a vertical plane by a horizontal axis, and weighted a little at one edge or rendered excentric so as to vibrate like a pendulum.  The poles of the magnets are applied near the side and edges of these plates, and then the number of vibrations, required to reduce the vibrating arc a certain constant quantity, noted.  In the first description of this instrument[A] it is said that opposite poles produced the greatest retarding effect, and similar poles none; and yet within a page of the place the effect is considered as of the same kind with that produced in iron.

  [A] Edin.  Phil.  Journal, 1825, p. 124.

250.  I had two such plates mounted, one of copper, one of iron.  The copper plate alone gave sixty vibrations, in the average of several experiments, before the arc of vibration was reduced from one constant mark to another.  On placing opposite magnetic poles near to, and on each side of, the same place, the vibrations were reduced to fifteen.  On putting similar poles on each side of it, they rose to fifty; and on placing two pieces of wood of equal size with the poles equally near, they became fifty-two.  So that, when similar poles were used, the magnetic effect was little or none, (the obstruction being due to the confinement of the air, rather,) whilst with opposite poles it was the greatest possible.  When a pole was presented to the edge of the plate, no retardation occurred.

251.  The iron plate alone made thirty-two vibrations, whilst the arc of vibration diminished a certain quantity.  On presenting a magnetic pole to the edge of the plate (247.), the vibrations were diminished to eleven; and when the pole was about half an inch from the edge, to five.