This electrostatic accessory is used with the Van de Graaff generator to show that an electric charge lies on the outer surface of a charged body. There is no charge on inner surfaces such as the inside of a pipe.
The Hollow Cylinder accessory has small lightweight balls suspended by thread which are located both above and inside a short length of tubing. When the tubing is charge, the balls touching its outside will immediately be repelled away from the cylinder. The two balls hanging inside the cylinder will be seen to be unaffected by the charge.