were on each
side of the plate, then the mutual action of the poles and the moving metal
was a maximum.

247. On suspending one magnet so that its axis was level with the plate,
and either pole opposite its edge, the revolution of the plate caused no
motion of the magnet. The electrical currents dependent upon induction
would now tend to be produced in a vertical direction across the thickness
of the plate, but could not be so discharged, or at least only to so slight
a degree as to leave all effects insensible; but ordinary magnetic
induction, or that on an iron plate, would be equally if not more
powerfully developed in such a position (251.).

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; an