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 Positive charge on a  positively charged
zinc plate was found to be further enhanced when it was illuminated by
ultraviolet light