We made sure to perform this experiment on a dry day because dry days tend to produce much better results. Excess moisture in the air will prevent a charge from building up because electricity is able to escape into humid air, or air that has a lot of water vapor in it.
Let’s think about why we constructed our electroscope the way we did. We’ll start with why we coiled the copper wire: we did this because we wanted to create a lot of surface area for electrostatic induction—the transfer of electrons from one surface to another without touching. The more area there is for electrons from our object to enter the wire, the faster energy will flow down into our electroscope. This is also why thicker wire is better: the bigger the radius of the wire, the more room electrons have to move.
It is necessary to have two pieces of foil or gold leaf touching so that they can repel each other when the electrons flow into them. Negative charges repel each other, so there must be two negatively charged pieces of foil for us to be able to visualize what is happening! Of course, electron transfer is still happening if there is one piece of metal, but we can’t see the particles themselves—we need to be able to see something that they do.
Electrons are transferred from one surface to another by physical contact, which is why it is important to rub the test materials with your hand or a piece of wool. By rubbing vigorously, you are transferring electrons from your hand or wool to the material you’re testing. Your hand and the wool become positively charged, while the test material becomes negatively charged. The electrons, which are negatively charged, can then travel through the copper wire of the electroscope down to the pieces of foil, which both become negatively charged. These negative charges push against each other, causing the pieces of foil to repel. Remember, while opposites attract, a negative-negative or positive-positive combinations of charges always repel!