The Experimental and Historical Foundations of Electricity - Unicamp

greatest discovery: 21
As we had no greater heights here, Mr. Wheler was desirous to
try whether we could not carry the electrick vertue horizontally. I
then told him of the attempt I had made with that design, but
without success, telling him the method and materials made use of,
as mentioned above. He then proposed a silk line to support the line
[of communication], by which the electrick vertue was to pass. I told
him it might do better upon the account of its smallness [that is,
Gray believed this could work better than in his original experiment
due to the small thickness of the silk thread in comparison with
the greater thickness of the packthread]; so that there would be less
[electric] vertue carried from the line of communication, with which,
together with the apt method Mr. Wheler contrived, and with the
great pains he took himself, and the assistance of his servants, we
succeeded far beyond our expectations.
The first experiment was made in the matted Gallery July 2, 1729,
about ten in the morning. About four feet [1.2 m] from the end of
the Gallery there was a cross line that was fixed by its ends to each
side of the Gallery by two nails; the middle part of the line was silk,
the rest at each end packthread; then the line to which the ivory ball
was hung, and by which the electrick vertue was to be conveyed to
it from the tube, being eighty feet and a half [24.5 m] in length, was
laidonthecrosssilkline, soasthat theball hungaboutninefeet[2.7
m] below it: Then the other end of the line [of communication] was
by a loop suspended on the glass cane, and the leaf-brass held under
the ball on a piece of white paper; when the tube being rubbed, the
ball attracted the leaf-brass, and kept it suspended on it for some
time.
A representation of this experiment can be found in Figure B.8 (a). A packthread
is connected to a glass tube and to an ivory ball at the other end. Below
the ball there are brass leaf. This packthread has an horizontal and a vertical
portion. At the junction of these two portions, it is supported above a stretched
silk thread. When Gray rubbed the glass, he observed the ball attracting the
brass leaf below it. This attraction did not happen for the situation of Figure
B.7. In this latter situation the string connected to the glass was suspended by
another packthread attached to the ceiling.
In Figure B.8 (b) we have a qualitative representation of the charges in this
experiment. In this case, the packthread is supported by an insulator, namely,
by the silk thread. There is no grounding here. The situation is like that of
Figure B.6 (b).
Here we have the fundamental discovery of conductors and insulators. For
conductors we have cork, ivory ball, wood, packthread, metal wires, etc. For
21 [Grah, pp. 26-27].
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