Electricity & Magnetism


PHOTOELECTRICITY consists of a sensitive gold foil electroscope, a zinc plate, a power supply, an ultraviolet lamp and a glass plate which blocks the lamp. Light shining on the zinc plate liberates electrons with considerable energy of motion, demonstrating the particle theory of light. The visitor is able to charge the electroscope with a probe which is attached to the power supply, but since the zinc plate is attached to the electroscope, and since the ultraviolet light liberates electrons from the zinc plate the electroscope slowly discharges. If the visitor blocks the light from the ultraviolet lamp the electroscope stays charged. If the electroscope receives a positive charge, (electrons are taken away) then the electroscope stays charged regardless of the ultraviolet light. This occurs because the lamp acts to reinforce the positive charge by removing electrons. .