Clean one face of the magnesium disc with either sandpaper or steel wool immediately before doing the experiment. Place the disc on the projection electroscope. A UV light source is set up fairly close to the disc.
If the disc is given a positive charge electrostatiscally by a Piezo charger and illuminated by the direct rays from the UV light source, it will remain charged. If the experiment is repeated with a negative charge on the disc, the electroscope will discharge quite rapidly. This shows that there is a net positive charge being produced in the magnesium disc. Further, if the experiment is repeated with a glass plate between the source and the disc, the effect will not be seen. This is due to the apparently clear glass filtering the UV light that produces the photoelectrons.