Hallwachs and Lenard studied how this photo current varied with
collector plate potential, and with frequency and intensity of incident light Hallwachs, in 1888, undertook the study further and connected a
negatively charged zinc plate to an electroscope He observed that the
zinc plate lost its charge when it was illuminated by ultraviolet light Further, the uncharged zinc plate became positively charged when it was
irradiated by ultraviolet light