The first
electrical engineer was probably William Gilbert who designed the versorium : a
device that detected the presence of statically charged objects. The versorium
is a needle constructed out of metal which is allowed to pivot freely on a
pedestal. The versorium is of a similar
construction to the magnetic compass, but is influenced by electrostatic rather
than magnetic forces. The needle is attracted to charged bodies brought near
it, turning towards the charged object.The needle turns to point at a nearby
charged object due to charges induced in the ends of the needle by the external
charge, through electrostatic induction.
The theory of
electrostatic induction says “A normal
uncharged piece of matter has equal numbers of positive and negative electrical
charges in each part of it, located close together, so no part of it has a net electric
charge. When a charged object is brought near an uncharged, electrically conducting object, such as a piece of metal,
the force of the nearby charge causes a separation of these charges”. For
example, if a positive charge is brought near the object the negative charges in the metal will be
attracted toward it and move to the side of the object facing it, while the
positive charges are repelled and move to the side of the object away from it.
This results in a region of negative charge on the object nearest to the
external charge, and a region of positive charge on the part away from it.
These are called induced charges.
If the external charge is negative, the polarity of the charged regions will be
reversed. Since this is just a redistribution of the charges that were already
in the object, the object has no net charge. This induction effect is
reversible; if the nearby charge is removed, the attraction between the
positive and negative internal charges cause them to intermingle again.
It should be kept in mind that only the negative charges in conductive
objects, the electrons, are free to move.