DEMONSTRATES: The emission of photoelectrons via the photoelectric effect.

EQUIPMENT: Black light, zinc plate mounted on electroscope, positive and negative charging materials.

DESCRIPTION: A zinc plate connected to an electroscope and charged positive will not discharge under the influence of ultraviolet radiation from the black light. When the plate is charged negative, however, UV light will discharge the plate, as indicated by the electrometer.

COMMENTS: Zinc plate must be polished before use to clear off oxide and expose zinc surfaces. Be cautious of what sign charge you have put on the plate.