The Experimental and Historical Foundations of Electricity - Unicamp

We have now a second criterion in order to say that a plastic or another
material is electrically neutral. The first criterion was presented in Experiment
2.1, namely, nottoattractlightweightbodies. Thesecondcriterionisthatofnot
producing orienting motion of a metal versorium when this material is placed
close to one of the legs. In the following experiments it is important to keep
one neutral plastic straw or ruler which neither attracts light bodies nor orients
metal versoria. This straw or ruler should not be rubbed in any event, as it will
be utilized as our neutral standard.
Fracastoro did not describe how he created the perpendiculo, which came
before Gilbert’s versorium. We can only speculate as to how he made it. One of
the goals of his book was the study of magnetism. He also wanted to distinguish
amber attraction from the attraction exerted by a natural magnet. Perhaps
he did rub a piece of amber in order to perform an electrical experiment and
casually noticed that it was capable of rotating a compass needle. As amber is
not magnetic, whether it is rubbed or not, he should have concluded that this
orientation of the compass was due to an electric attraction, analogous to the
attraction of lightweight bodies by the rubbed amber. He could have decided
then to make metal needles analogous to magnetic needles, but not magnetized.
They would rotate toward a rubbed piece of amber, but not toward a magnet
(assuming needles made of copper or silver, but not made of iron nor steel). He
may thus have created the first artificial instrument for the study of electricity.
Experiment 3.2
We move a magnet near a versorium. We see that only versoria made of
steel, iron, nickel, or other ferromagnetic materials rotate and direct themselves
toward the magnet. Versoria made of other materials are not affected by the
magnet. We also see that several metals, like copper and aluminum, are not
affected by the magnet. The same happens with the majority of substances
(paper, plastic, wood, etc.)
With this experiment we can distinguish magnetic interaction from electric
interaction, as we did earlier with Experiments 2.6 and 2.7, but now with a
greater precision.
3.4 Is it Possible to Map the Electric Force?
Is it possible to map the electric force exerted by a rubbed plastic body? Can
we visualize in which direction a long rubbed straw will attract a piece of paper
placed nearby? In this Section we answer this question.
In the following experiments we can use several versoria simultaneously. We
can also use a single versorium which is placed alternately in several positions
around the rubbed body for each experiment. In the next figures we show
several versoria at the same time. It is best to utilize small versoria, such as the
ones made of small paper fasteners placed on pins. These pins can be attached
to several corks, or they can all be stuck into a Styrofoam board. Initially we
work only with metal versoria.
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