Alternate charge resides on outside of conductor

This demonstration is provided to illustrate how electrical fields cannot penetrate a conductor. A hollow cylinder, open at both ends, is mounted on an insulating stand. Two pith balls hang down in the middle of the cylinder and two are mounted from a peg attacked to the top and allowed to hang on the outside of the cylinder. Bringing a source of charge nearby will dramatically move the two balls on top but the balls inside will not respond. A camera can be provided to make the demonstration obvious to a large classroom.