magnet and B
for the second period, withdrawn and restored to its first place for the
third period, and so on, but with no effect on the needle.

1713. In these experiments _shell-lac_ in plates 0.9 of an inch in
thickness, _sulphur_ in a plate 0.9 of an inch in thickness, and _copper_
in a plate 0.7 of an inch in thickness were used without any effect. And I
conclude that bodies, contrasted by the extremes of conducting and
insulating power, and opposed to each other as strongly as metals, air, and
sulphur, show no difference with respect to magnetic forces when placed in
their lines of action, at least under the circumstances described.

1714. With a plate of iron, or even a small piece of that metal, as the
head of a nail, a very different effect was produced, for then the
galvanometer immediately showed its sensibility, and the perfection of the
general arrangement.

1715. I arranged matters so that a plate of _copper_ 0.2 of an inch in
thickness, and ten inches in diameter, should have the part near the edge
interposed between the magnet and the core, in which situation it was first
rotated rapidly, and then held quiescent alternately, for periods according
with that required for the swinging of the needle; but not the least effect
upon the galvanometer was produced.

1716. A plate of shell-lac 0.6 of an inch in thickness was applied in the
same manner, but whether rotating or not it produced no effect.

1717. Occasionally the plane of rotation was directly across the magnetic
curve: at other times it was made as oblique as possible; the direction of
the rotation being also changed in different experiments, but not the least
effect was produced.

1718. I now removed the helices with their soft iron cores, and replaced
them by two _flat helices_ wound upon card board, each containing forty-two
feet of silked copper wire, and having no associated iron. Otherwise the
arrangement was as before, and exceedingly sensible; for a very slight
motion of the magnet between the helices produced an abundant vibration of
the galvanometer needle.

1719. The introduction of plates of shell-lac, sulphur, or copper into the
intervals between the magnet and these helices (1713.), produced not the
least effect, whether the former were quiescent or in rapid revolution
(1715.). So here no evidence of the influence of the intermediate particles
could be obtained (1710.).

1720. The magnet was then removed and replaced by a flat h