Charge and Carry
Store up an electric charge, then make sparks.
Are you tired of electrostatic experiments that just won't work? This experiment will produce a spark that you can feel, see, and hear. You rub a Styrofoam plate with wool to give it a large electric charge. Then you use the charged Styrofoam to charge an aluminum pie pan. The entire apparatus for charging the aluminum plate is called an electrophorus, which is Greek for charge carrier. An even larger charge can be stored up in a device called a Leyden jar, made from a plastic film can.

For the Electrophorus:

For the Leyden jar:

(15 minutes or less)

Electrophorus:

Tape or hot-glue the Styrofoam cup to the middle of the inside of the pie plate. (Most household glues won't work because they dissolve Styrofoam.) Place the pie pan on top of the upside-down Styrofoam plate or a piece of acrylic plastic.

Leyden Jar:


Push the nail through the center of the lid of the film can. Wrap aluminum foil around the bottom two-thirds of the outside of the film can. You may tape the aluminum foil in place. Fill the film can almost full with water. Snap the lid onto the can. The nail should touch the water.

(30 minutes or more)

Rub the Styrofoam plate with the wool cloth. If this is the first time you are using the Styrofoam in an electrostatic experiment, rub it for a full minute.

To charge the pie pan follow the next steps exactly:

1. Place the pie pan on top of the charged Styrofoam plate.

2. Briefly touch the pie pan with your finger. You may hear a snap and feel a shock.

3. Remove the pie pan using only the insulating Styrofoam cup (see photo). You may have to hold the Styrofoam plate down with your other hand.

The pan is now charged.

Discharge the pan by touching it with your finger. You will hear a snap, feel a shock, and, if the room is dark, see a spark. To make the largest spark, have the pie plate at least one foot away from the Styrofoam plate. You can also discharge the pie pan through a neon glow tube. Hold one of the two metal leads of the tube in your fingers and touch the other lead to the pie pan. The electric spark will go through the neon and make a flash that is easily visible. After charging the Styrofoam once, you can charge the pie pan several times. The pie pan is portable and can be used for many electrostatic experiments.

Charge the Leyden jar by touching the charged pie pan to the nail while holding the Leyden jar by its aluminum foil covering. You can make several charge deliveries by recharging the pan before touching it to the nail. Discharge the jar by touching the aluminum foil with one finger and the nail with another. Watch for a spark.

When you rub the Styrofoam plate with a wool cloth, you charge it negatively. That's because the Styrofoam 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 the Electroscope Snack, you can determine whether the Styrofoam is positively or negatively charged. Styrofoam is an insulator; it will hold its charge until it is discharged by current leaking into the air or along a moisture film on the surface of the Styrofoam

When you place the pie pan on the Styrofoam, the electrons on the Styrofoam repel the electrons on the pan. Since the electrons can't leave the pie pan because it is completely surrounded by insulating air and Styrofoam, the pan retains its neutral charge. If you touch the pie pan while it is near the Styrofoam, 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 though 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. If you bring the positive 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.

The low-pressure neon gas in a neon glow tube is easier to ionize than air that is at atmospheric pressure. If you discharge the pan through a neon glow tube, the spark will make a bigger flash of light.

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 positive center separated from a 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.

The Leyden jar is the forerunner of the modern-day capacitor. It was invented in 1745 at the University of Leyden by Pieter Van Musschenbroeck. Early Leyden jars were larger than a plastic film can and could hold more charge. The inventor discharged one through himself and wrote, "My whole body was shaken as though by a thunderbolt." At another time, a Leyden jar was discharged through 700 monks who were holding hands. The charge caused them to simultaneously jump slightly off the ground.

To give the Styrofoam plate a positive charge, try rubbing it with a plastic bread bag. Try rubbing it with other cloths, too. Try charging the Leyden jar in reverse. That is, while holding the nail, touch the aluminum foil with the pan.