to east, the marked pole went
eastward.

169. The deflections of the magnetic needle thus obtained with a brass ball
are exactly in the same direction as those observed by Mr. Barlow in the
revolution of the iron shell; and from the manner in which iron exhibits
the phenomena of magneto-electric induction like any other metal, and
distinct from its peculiar magnetic phenomena (132.), it is impossible but
that electric currents must have been excited, and become active in those
experiments. What proportion of the whole effect obtained is due to this
cause, must be decided by a more elaborate investigation of all the
phenomena.

170. These results, in conjunction with the general law before stated
(114.), suggested an experiment of extreme simplicity, which yet, on trial,
was found to answer perfectly. The exclusion of all extraneous
circumstances and complexity of arrangement, and the distinct character of
the indications afforded, render this single experiment an epitome of
nearly all the facts of magneto-electric induction.

171. A piece of common copper wire, about eight feet long and one twentieth
of an inch in thickness, had one of its ends fastened to one of the
terminations of the galvanometer wire, and the other end to the other
termination; thus it formed an endless continuation of the galvanometer
wire: it was then roughly adjusted into the shape of a rectangle, or rather
of a loop, the upper part of which could be carried to and fro over the
galvanometer, whilst the lower part, and the galvanometer attached to it,
remained steady (Plate II. fig. 30.). Upon moving this loop over the
galvanometer from right to left, the magnetic needle was immediately
deflected; upon passing the loop back again, the needle passed in the
contrary direction to what it did before; upon repeating these motions of
the loop in accordance with the vibrations of the needle (39.), the latter
soon swung through 90 deg. or more.

172. The relation of the current of electricity produced in the wire, to
its motion, may be understood by supposing the convolutions at the
galvanometer away, and the wire arranged as a rectangle, with its lower
edge horizontal and in the plane of the magnetic meridian, and a magnetic
needle suspended above and over the middle part of this edge, and directed
by the earth (fig. 30.). On passing the upper part of the rectangle from
west to east into the position represented by the dotted line, the marked
pole o