ith the revolving disc (94.).

108. On the conductors being held against the ends of the plates, and the
latter then passed between the magnetic poles, in a direction transverse to
their length, the same effects were produced (fig. 22.). The parts of the
plates towards the end may be considered either as mere conductors, or as
portions of metal in which the electrical current is excited, according to
their distance and the strength of the magnet; but the results were in
perfect harmony with those before obtained. The effect was as strong as
when the conductors were held against the sides of the plate (101.).

109. When a mere wire, connected with the galvanometer so as to form a
complete circuit, was passed through between the poles, the galvanometer
was affected; and upon moving the wire to and fro, so as to make the
alternate impulses produced correspond with the vibrations of the needle,
the latter could be increased to 20 deg. or 30 deg. on each side the magnetic
meridian.

110. Upon connecting the ends of a plate of metal with the galvanometer
wires, and then carrying it between the poles from end to end (as in fig.
23.), in either direction, no effect whatever was produced upon the
galvanometer. But the moment the motion became transverse, the needle was
deflected.

111. These effects were also obtained from _electro-magnetic poles_,
resulting from the use of copper helices or spirals, either alone or with
iron cores (34. 54.). The directions of the motions were precisely the
same; but the action was much greater when the iron cores were used, than
without.

112. When a flat spiral was passed through edgewise between the poles, a
curious action at the galvanometer resulted; the needle first went strongly
one way, but then suddenly stopped, as if it struck against some solid
obstacle, and immediately returned. If the spiral were passed through from
above downwards, or from below upwards, still the motion of the needle was
in the same direction, then suddenly stopped, and then was reversed. But on
turning the spiral half-way round, i.e. edge for edge, then the directions
of the motions were reversed, but still were suddenly interrupted and
inverted as before. This double action depends upon the halves of the
spiral (divided by a line passing through its centre perpendicular to the
direction of its motion) acting in opposite directions; and the reason why
the needle went to the same side, whether the spiral passe