15.1. Static Electricity http://www.ck12.org
FIGURE 15.5
Conduction charges neutral coin.A reasonable question at this point would be: How do we know both coins are charged? For that matter, how could
we have known the first coin was charged?
The Electroscope and Charging by Induction
The electroscope is a device that can detect the presence of excess charge on an object through a process known as
induction. The first gold-leaf electroscope was constructed in 1787 by Abraham Bennet (1749-1799),Figure15.6.
He used the gold leaf for several reasons. Gold is an excellent conductor, gold leaf can be made very thin and
light-weight, and gold is very malleable.
FIGURE 15.6
Abraham Bennet, inventor of the electroscope.Consider the coin inFigure15.7. In order to ensure that the coin retains its charge (assuming we were able to apply
the charge in the first place) we suspend it by an insulating thread,Figure15.7.
An electroscope,Figure15.8, is typically contained within glass. A metal rod or disk (see figure) protrudes through
the top of the glass. Attached to the metal disk (in this case) within the glass is a very thin, light-weight folded foil–
the “leaves” of the electroscope.
When a charged object is brought close to the top of the disk, the electrons in the rod and the leaves of the
electroscope will either move toward the charged object or away from the object. If the object is positively charged
as in the case of the coin inFigure15.8, the electrons are attracted toward the coin. This produces a surplus of
positive charges on the two leaves. Since both leaves have the same charge, they repel each other. We can be certain