Electroscopes
An electroscope is an
instrument for detecting the presence of static electricity. It
consists of two thin metal leaves suspended from a metal hook. When the
hook is brought near a source of static electricity, some of
the electrons in the hook are pushed to the leaves (if the source is
negative) or pulled up to the hook from the leaves (if the source is
positive). Either way, the leaves are now charged the same way as each
other and so they repel each other. The amount they open up is
proportional to the charge of the source (if the sources are
always held at the same distance from the hook).