The Electroscope

The electroscope was an early instrument used for

detecting charge on bodies
identifying the sign of the charge
roughly estimating the amount of charge on a body.

It was used a good deal in the early days to discover the qualitative concepts about electric charges that we have discussed so far.

The main part of an electroscope is a very thin, very light, metal foil, often made of gold, that hangs from the bottom of a metal rod, next to a piece of metal. (Sometimes one can also see electroscopes that have a pair of metal foils.) The upper end of the metal rod terminates in a cap. The rod is enclosed in a metal box, through an insulating stopper, keeping the cap outside. The bodies to be tested are brought near the cap. Two of the sides of the metal box have glass windows, so that one can see what is happening to the metal foils (also called leaves). The metal box is grounded, i.e. connected to the Earth.

In the picture on the left you can see an electroscope that is typically used in our introductory physics lecture demonstration setup. On the left is is uncharged, and on the right a very minute amount of charge has been put on it. You can see that in this case the gold foil pulls away from the metal plate. Why this repulsion? This is the next topic...