The Experimental and Historical Foundations of Electricity - Unicamp

Given a constant external torque, it is easier to rotate a lighter weight mobile
part of a versorium than a heavier one. This means that a light versorium has
a greater sensitivity than a heavy one.
Although Gilbert built only metal versoria, they can be made of different
materials: metal, plastic, thin cardboard, dry straw, wood, etc. Initially we
will work only with metal versoria, which we will call simply versoria. When
the spinning needle is made of plastic, paper, or of another non-metal material,
we will call the system a plastic versorium, a paper versorium, or the appropriate
name. In this way we will be able to distinguish these versoria from the
versorium utilized by Gilbert.
3.3 Experiments with the Versorium
Experiment 3.1
We bring a neutral plastic near a metal versorium, without bringing them
into contact. Nothing happens (Figure 3.10).
(a)
(b)
Figure 3.10: (a) A metal versorium points in an arbitrary direction when it is
far away from a neutral piece of plastic. (b) The versorium remains at rest when
the neutral plastic is brought near it.
We rub another piece of plastic and repeat the experiment. In this case we
observe that the metal versorium is oriented by the rubbed plastic, pointing
toward it (Figure 3.11). The same happens with a wood versorium and a paper
versorium.
This experiment shows that the rubbed plastic affects nearby bodies, as we
saw in Experiment 2.1. But there are two main differences between these two
experiments. The first difference is that in Experiment 2.1 there was movement
of the small pieces of paper. In the present experiment only the versorium
changes direction, while its center remains at rest above the pin. The second
differenceisthattheversoriummovesmoreeasilythanthepiecesofpaper. That
is, some rubbed objects cannot attract light bodies to themselves. However,
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