1. Before doing the experiment(s), scrub both sides of the zinc or galvanized steel plate with the steel wool in order to remove the surface oxide from both sides. The light must be able to extract electrons from bare zinc, and they must be able to be readily transported to the electroscope. Scraping it once before the experiment seems to do the trick. Place the zinc plate on top of the electroscope’s circular metal plate.

2. There are two different ways to charge the electroscope with a negative charge (excess electrons). One can either charge up the plastic rod with the wool and scrape it on the zinc plate (it can be tricky to successfully charge the electroscope this way since you can’t hold the zinc plate onto it as this will ground it). Alternatively, you can charge up the glass rod with the wool (this makes a positive charge). Next, bring the charged rod close to the electroscope without touching the rod to the plate (the needle should deflect due to polarization). Keeping the rod close, touch the electroscope with your finger. Now remove your finger from the electroscope. It should be negatively charged. If the needle is not deflected after you’ve removed the rod, you’ll have to try again.

3. Once you have zinc plate / electroscope negatively charged, notice how the needle is fixed. And yet there are electromagnetic waves from the room that are hitting the plate. Classical E&M says that if those waves were strong enough, they should be able to knock the electrons off of the plate, discharging the scope. You can try increasing the brightness of light in the room, and the leaf won’t budge. You can try turning on the bright incandescent bulb, and moving it close to the plate (without touching it), and the leaf should not budge.

4. Now plug in the UV source. Taking care not to point it at anything other than the plate (you can use protection goggles, but this is not really necessary if you’re careful with where you point it). Turn it on. The needle’s deflection should promptly decrease. If it doesn’t go all the way to 0, try moving the lamp around slightly to shine the light on the whole plate.

5. One more cool thing you can do is to charge the electroscope positively instead of negatively (either scrape the positively charged rod directly onto the zinc, or use charging by induction with

the plastic rod). Repeat the experiment. The UV light no longer removes charge from the electroscope.