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. A 1/8" glass plate inserted into the light from the UV bulb prevents passage of UV and the discharge ceases. Removing the glass plate allows the discharge to continue.
NOTE: This arrangement does not work to show that a positive zinc plate will NOT be discharged by the UV source. If you wish to show the effect with both positive and negative charge on the plate you must use the carbon arc lamp, Demonstration P2-02.