In this experiment the arc lamp acts both as a source of ultraviolet radiation for discharging the zinc plate and as a bright light to shadow project the apparatus.
A zinc plate connected to an electroscope and charged positive will not discharge under the influence of ultraviolet radiation. When the plate is charged negative, however, light from the arc lamp, which contains much UV, will discharge the plate, as indicated by the electrometer. A 1/8" glass plate inserted into the light from the arc lamp prevents passage of UV and the discharge ceases. Removing the glass plate allows the discharge to continue.