To demonstrate that the electric field within a closed conducting surface is zero.
Wire screen Faraday cage with pith balls and charging materials.
When the cage is charged, the aluminum foil "pith balls" move away from the outside of the cage, indicating the presence of an electric field. However, on the inside of the cage the pith balls do not move away from the wire cage, indicating that there is no charge and therefore no electric field on the inside of the conducting surface.◙
Zvi Geller and Esther Bagno, Does Electrostatic Shielding Work Both Ways?, TPT 32, 20-22 (1994).