Figure 3. Gold-leaf electroscope.
A gold-leaf electroscope is a device that can be used to test for electrical potential and charge and the sign of the charge if it is first given a charge of known sign.
The basic construction consists of an earthed metal box with glass windows, as shown in Figure 3. A metal rod is held in place insulated from the box. On the top of the rod there is a metalic cap and at the other end of rod a strip of gold-leaf is attached part the way up the rod.
When the cap is touched with a charged object, the charge flows down the metal rod and the gold leaf. Since both conductors will have an excess of the same type of charge, the repulsion between like charges causes the gold-leaf to be deflected from the end of the rod.
If the electroscope is first charged with a positive charge and a positive charge is brought near to the cap of the electroscope, the gold leaf will be deflected more because there is a larger potential difference between the leaf and the case. While if a negative charge is brought close to the cap of the electroscope, the gold leaf will fall. No charge will be lost, but the potential difference between the case and the leaf will be reduced.
Faradays Ice Pail Experiment
Michael Faraday used a metal ice pail as a conducting object to study how charges distribute themeselves when a charged object was placed inside the pail. The experiment, starts with a neutal ice pail, connected to an electroscope. A positively charged sphere is brought over to the metal container. As the charged sphere is moved inside the metal container (but without touching the sides of the ice pail) the sphere induces a negative charge equal to the positive charge on the sphere on the inside of the metal container. On the outside of the metal container, there is also an induced positive charge of equal magnitude as that on the sphere, as indicated by the gold-leaf electroscope.
If the sphere is touched on the inside of the metal container the charge on the sphere flows to the outside of the metal container. The experiment demonstrates that if a conducting path is provided, charge on any metal object will reside on the outer surface.
If a positively charged sphere is brought close to the outside surface of the pail, the charges will redistribute themselves on the outside of the pail so as to cancel the electric field inside the pail.
