Purpose:Demonstrate that the electric field within a closed surface is zero.
Description:Charge the two wire strainers with triboelectric materials (J1-01), which are connected electrically to the electroscope indicator. With the two meshes separated, the electroscope deflects. When they are placed together, forming a closed sphere, the electroscope deflection is zero, indicating zero electric field within the closed cage. The charge that was originally on the electroscope has flowed to the outside of the conductor. Note that if you then re-open the two hemispheres, the electroscope again deflects as the charge flows back into it.
An alternative way to use this demo is to bring the wire strainers near to a charged van de Graaff, first open, then closed. The difference in this case is that the electric field being shielded is fully external to the device, in contrast to the first method where it was an internally generated field.
This is best used with a light source to project the shadow of the electroscope needle through the mesh onto the wall.