The Hollow Sphere is used to show that electrostatic charges lie only on the outside surface of a charged body. The unit may be charged by touching it to the dome of a Van de Graaff Generator. It is then probed by means of a Proof Plane and an electroscope.
When the Proof Plane is touched to the electroscope, it will be clearly seen that a charge has been acquired. However, when the Proof Plane is carefully inserted through the hole in the top of the Hollow Sphere, touched to the inside surface, and then brought to the electroscope, no charge will be found.
Sphere diameter: 10 cm
Sphere opening: 4 cm
Height with base: 26 cm