When you rub the foam plate with a wool cloth, you charge it negatively. That’s because the foam attracts electrons from the cloth. Often, a plate fresh from the package will start with a positive charge. If it does, you will have to rub the plate long enough to cancel this initial charge before you can begin building a sizable negative charge. By using an electroscope (such as the one you can build with our Electroscope activity), you can determine whether the foam is positively or negatively charged. Plastic foam or StyrofoamTM is an insulator; it will hold its charge until it is discharged by current leaking into the air or along a moisture film on its surface.
When you place the pie pan on the foam plate, the electrons on the foam repel the electrons on the pan. Since the electrons can’t leave the pie pan because it is completely surrounded by insulating air and foam, the pan remains neutral. If you touch the pie pan while it is near the foam plate, the mobile electrons will be pushed off the pan and onto you. The electrons make a spark as they jump a few millimeters through the air to reach your finger. The air in the spark is ionized as the moving electrons knock other electrons off air molecules. The ionized air emits light and sound. You can also feel the flow of electrons through your finger.
After the electrons leap to your finger, the pan has a positive charge. Physicists say the pan has been charged by induction. You can carry the positively charged pan around by its handle and carry the positive charge to other objects. For example, if you bring the positively charged pan near your finger again, or near any object that can be a source of electrons, the pan will attract electrons, creating a second spark.
When you touch a positively charged pie pan to the nail on the Leyden jar, electrons from the nail flow onto the pie pan. The resulting positive charge on the nail attracts electrons from your body through your hand onto the aluminum foil of the jar. The Leyden jar will then have a positively charged center separated from the negative foil outside by the insulating plastic of the film can. If you touch one finger to the foil and bring another finger near the nail at the center of the Leyden jar, a spark will jump, as the negative charges are attracted through you to the positive nail.
The beauty of the Leyden jar is that it can store charges from several charged pie pans, thus building up to a larger, more visible, more powerful (and more painful) spark.