This is an instrument for detecting
and measuring static electricity or voltage.
A metal disc is connected to a narrow metal
plate and a thin piece of gold leaf is fixed to the plate. The whole of this part of the
electroscope is insulated from the body of the instrument. A glass front prevents air draughts
but allows you to watch the behaviour of the leaf.
When a charge is put on the disc at the top it
spreads down to the plate and leaf. This means that both the leaf and plate will have the
same charge. Similar charges repel each other and so the leaf rises away from the plate - the
bigger the charge the more the leaf rises.
The leaf can be made to fall again by
touching the disc - you have earthed the electroscope. An earth terminal prevents the case
from becoming live. The electroscope can be charged in two ways:
(a) by contact -
a charged rod is touched on the surface of the disc and some of the charge is transferred to
the electroscope. This is not a very effective method of charging the
electroscope.
(b) by induction - a charged rod is brought up to the disc and then
the electroscope is earthed, the rod is then removed.
The two methods give the gold
leaf opposite charges.
The following diagrams show you how the
charges spread over the plate and gold leaf in different
conditions.
