By R. Suhrmann, Hanover

1. Definition of the external photoelectric effect

When a negatively charged metal plate, connected to an electroscope, is illuminated with the light from a carbon arc lamp, the leaves of the electroscope collapse. This means that the metal plate loses its negative charge due to the light exposure. However, the metal plate does not release a positive charge when exposed to light. This effect, first demonstrated by Hallwachs in 1887, shows that the plate loses its negative charge upon illumination; it emits electrons, which, as we know from Lenard's investigations, transfer to the network and are then discharged to the ground.