27. A welded ring was made of soft round bar-iron, the metal being
seven-eighths of an inch in thickness, and the ring six inches in external
diameter. Three helices were put round one part of this ring, each
containing about twenty-four feet of copper wire one twentieth of an inch
thick; they were insulated from the iron and each other, and superposed in
the manner before described (6.), occupying about nine inches in length
upon the ring. They could be used separately or conjointly; the group may
be distinguished by the letter A (Pl. I. fig. 1.). On the other part of the
ring about sixty feet of similar copper wire in two pieces were applied in
the same manner, forming a helix B, which had the same common direction
with the helices of A, but being separated from it at each extremity by
about half an inch of the uncovered iron.

28. The helix B was connected by copper wires with a galvanometer three
feet from the ring. The helices of A were connected end to end so as to
form one common helix, the extremities of which were connected with a
battery of ten pairs of plates four inches square. The galvanometer was
immediately affected, and to a degree far beyond what has been described
when with a battery of tenfold power helices _without iron_ were used
(10.); but though the contact was continued, the effect was not permanent,
for the needle soon came to rest in its natural position, as if quite
indifferent to the attached electro-magnetic arrangement. Upon breaking the
contact with the batterry, the needle was again powerfully deflected, but
in the contrary direction to that induced in the first instance.

29. Upon arranging the apparatus so that B should be out of use, the
galvanometer be connected with one of the three wires of A (27.), and the
other two made into a helix through which the current from the trough (28.)
was passed, similar but rather more powerful effects were produced.