The Experimental and Historical Foundations of Electricity - Unicamp

the disk.
plastic straw
disk of thin cardboard
Figure 7.5: Coulomb’s proof plane. This is also a collector but will be referred
to as a proof plane for clarity when describing the following experiments.
These charge collectors will be used in some of the experiments described
here.
7.3 The Electric Polarization of Conductors
Experiment 7.3
We saw in Experiment 6.12 that thin cardboard is a conductor, while plastic
is an insulator. We use this fact to build an insulated conductor: A horizontal
thin cardboard disk 15 cm in diameter is supported by four vertical plastic
straws attached to appropriate supports, such as the supports of the electric
pendulums. We place three Coulomb proof planes on the top of the disk, aligned
along a diameter of the disk, one at the center and the other two close to the
opposite edges. We call them 1, 2, and 3, with the 2nd proof plane at the center
of the disk. Initially we touch the disk with our finger in order to discharge it.
We now begin the experiment to study the distribution of charges on this disk
in the presence of other nearby charged bodies. To do this, we first charge two
electroscopes, one positively and the other negatively. We place them far away
from one another and from this horizontal disk. We also will utilize a third
discharged electroscope.
We charge a straw negatively along its entire length when we rub it in our
hair. It should then be stood vertically on an appropriate support. The middle
of this straw should be at the same height as the horizontal plane of the disk.
The rubbed straw is brought near the disk, without touching it, close to proof
plane1(Figure7.6). Theywillthenbeinthefollowingsequence: negativestraw,
proof planes 1, 2, and 3, respectively. The negative straw should be about 2 cm
from the closest edge of the disk. We remove proof plane 2 and bring it close to
the discharged electroscope. Nothing happens, indicating that it is electrically
neutral. We can put it back in its original place. We now remove proof plane 1
and slowly bring it near the discharged electroscope, without making contact.
The strip of this electroscope is attracted by this proof plane, indicating that the
proof plane is charged. We then slowly bring it near the negative and positive
electroscopes, asalwayspreventingthemfromcomingintocontact. Thecharged
proof plane attracts the strip on the negative electroscope and repels the strip
on the negative electroscope. From these facts we conclude that proof plane
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