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The gold leaf electroscope

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.

schoolphysics: Gold leaf electroscope animation

To see an animation of the effect of a flame and a radioactive source on a charged gold leaf electroscope click on the animation link.

© Keith Gibbs 2020