l discs superposed; the discs are to be metallically connected,
alternately at the edges and at the centres, by means of mercury; and are
then to be revolved alternately in opposite directions, i.e. the first,
third, fifth, &c. to the right hand, and the second, fourth, sixth, &c. to
the left hand; the whole being placed so that the discs are perpendicular
to the dip, or intersect most directly the magnetic curves of powerful
magnets. The electricity will be from the centre to the circumference in
one set of discs, and from the circumference to the centre in those on each
side of them; thus the action of the whole will conjoin to produce one
combined and more powerful current.

159. I have rather, however, been desirous of discovering new facts and new
relations dependent on magneto-electric induction, than of exalting the
force of those already obtained; being assured that the latter would find
their full development hereafter.

* * * * *

160. I referred in my former paper to the probable influence of terrestrial
magneto-electric induction (137.) in producing, either altogether or in
part, the phenomena observed by Messrs. Christie and Barlow[A], whilst
revolving ferruginous bodies; and especially those observed by the latter
when rapidly rotating an iron shell, which were by that philosopher
referred to a change in the ordinary disposition of the magnetism of the
ball. I suggested also that the rotation of a copper globe would probably
insulate the effects due to electric currents from those due to mere
derangement of magnetism, and throw light upon the true nature of the
phenomena.

[A] Christie, Phil. Trans. 1825, pp. 58, 347, &c. Barlow, Phil. Trans.
1825, p. 317.

161. Upon considering the law already referred to (114.), it appeared
impossible that a metallic globe could revolve under natural circumstances,
without having electric currents produced within it, circulating round the
revolving globe in a plane at right angles to the plane of revolution,
provided its axis of rotation did not coincide with the dip; and it
appeared that the current would be most powerful when the axis of
revolution was perpendicular to the dip of the needle: for then all those
parts of the ball below a plane passing through its centre and
perpendicular to the dip, would in moving cut the magnetic curves in one
direction, whilst all those parts above that plane would intersect them in
the other di