Shuttling Ball Experiment

The shuttling ball experiment shows that a current is a flow of charge. A light ball with a conducting coating is hung from an insulating thread between two vertical metal plates, which are connected to a high-voltage supply.When the ball touches the negative plate, electrons flow onto it and it becomes negatively charged. It is then repelled by the negative plate and attracted towards the positive plate. On touching the positive plate it gives up electrons, becomes positively charged, and is repelled back towards the negative plate.The ball therefore shuttles back and forth, carrying a small charge q across the gap on each crossing. If it makes n crossings per second, the current in the circuit is I = nq, which can be measured with a sensitive ammeter or picoammeter.Increasing the p.d. between the plates increases both the charge on the ball and the force on it, so it shuttles faster and the current increases. Moving the plates closer together increases the field strength E = V/d and has a similar effect.