The Experimental and Historical Foundations of Electricity - Unicamp

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Figure 3.13: Orientation of versoria toward a horizontal straw which has been
rubbed along its entire length.
We now repeat these experiments using two rubbed straws standing vertically.
In this case the configuration of the versoria is shown in Figure 3.14.
The circles with the letters � represent the rubbed portions of the straws which
are at the same level as the versoria. This configuration indicates a vectorial
addition of the torques exerted by each plastic straw on the versoria. Vectors
add to produce the resultant by the parallelogram rule.
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Figure 3.14: Orientation of the versoria due to two rubbed plastic straws.
It is of historical interest that Gilbert did not employ a versorium in order
to map the electric force as we are doing here. But he did use magnetized
compass needles in order to map the magnetic force of a magnet. In Figure
3.15 we show the results he obtained for cylindrical and spherical magnets. 12
The spherical magnet orients the compasses analogously to the orientation of
compassesabovetheEarth,whichpointtowardtheNorth-Southmagneticpoles.
In other words, the small spherical magnet also has two poles, which are the
points upon the surface of the sphere close to which the compasses remain
perpendicular to the surface of the sphere, pointing toward the center of the
sphere. It is possible to draw the magnetic meridians over the surface of this
sphere. They are circles connecting these two poles, with the centers of the
12 [Gil78, pp. 10 and 82].
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