The strong positive charge on the glass attracts the electrons in the wire on the top brush. These electrons spray from the sharp points in the brush, and charge the air. The air is repelled from the wire, and attracted to the glass.

But the charged air can't get to the glass, because the rubber band is in the way. The charged air molecules hit the rubber, and transfer the electrons to it.

The rubber band travels down to the bottom brush. The electrons in the rubber push on the electrons in the wire of the bottom brush. The electrons are pushed out of the wire, and into whatever large object we have attached to the end of the wire, such as the earth, or a person.

The sharp points of the bottom brush are now positive, and they pull the electrons off of any air molecules that touch them. These positively charged air molecules are repelled by the positively charged wire, and attracted to the electrons on the rubber band. When they hit the rubber, they get their electrons back, and the rubber and the air both lose their charge.

The rubber band is now ready to go back up and steal more electrons from the glass tube.

The top brush is connected to the inside of the soda can. It is positively charged, and so attracts electrons from the can. The positive charges in the can move away from one another (they are the same charge, so they repel, just like electrons). The positive charges collect on the outside of the can, leaving the neutral atoms of the can on the inside, where they are always ready to donate more electrons.

The effect is to transfer electrons from the soda can into the ground, using the rubber band like a conveyor belt. It doesn't take very long for the soda can to lose so many electrons that it becomes 12,000 volts more positive than the ground.

When the can gets very positive, it eventually has enough charge to steal electrons from the air molecules that hit the can. This happens most at any sharp points on the can. If the can were a perfect sphere, it would be able to reach a higher voltage, since there would be no places where the charge was more concentrated than anywhere else.

If the sphere were larger, an even higher voltage could be reached before it started stealing electrons from the air, because a larger sphere is not as "sharp" as a smaller one.

The places on our soda can where the curves are the sharpest are where the charge accumulates the most, and where the electrons are stolen from the air.

Air ionizes in an electric field of about 25,000 volts per inch. Ionized air conducts electricity like a wire does. You can see the ionized air conducting electricity, because it gets so hot it emits light. It is what we call a spark.

Since our generator can draw sparks that are about a half inch long, we know we are generating about 12,500 volts.

Troubleshooting

If you aren't detecting any high voltage (no sparks, doesn't attract hair or paper) then you might try some of these suggestions.

• Try a different type of rubber band. Some are slightly conductive, which at 12,000 volts means conductive enough to leak all the current you have so carefully built up. Have a supply of many different types of rubber band to try.

• Make sure everything is very clean. Dirt and grease can be slightly conductive, and that will be enough to make the device fail.

• Make sure the top brush is touching the metal of the can.

Some cans have a plastic coating inside. Scrape it off (or burn it off) to make a better connection.

• Make sure there are no sharp points extending outside the can. It is OK to have sharp points pointing inside the can, from the cut part of the top. Sharp points cause corona losses.

• Make sure the brushes are not touching the rubber band. This will put a coating of copper on the rubber, and make it conductive.

• Make sure you have a good ground connection.

• Make sure the motor is spinning fast.

Check our Message Board for more ideas, and be sure to search for "VDG" and "rubber band" to get all of the messages. Since people can't spell Van de Graaff, you may want to try various spellings.

Some fun with the Van de Graaf generator

One of the fun things to do with a Van de Graaff generator is to show how like charges repel.

We take a paper napkin, and cut thin strips of the lightweight paper. We then tape the ends of the paper together at one end, and tape that end onto the Van de Graaf generator.

The effect will look somewhat like long hair cascading down the soda can.

Now turn the Van de Graaff generator on. The thin strips of paper all get the same charge, and start to repel from one another. The effect is "hair raising". The strips start to stand out straight from the can, like the hair on the back of a scared cat.

A high voltage ion motor

This motor is very simple to build, and goes together in a few minutes. All you need is two pieces of wire, the small metal cap from the fuse we took apart in the previous project, and some cellophane tape.

The motor creates an ion wind that spins it around like a helicopter.

First, take one piece of wire (a straightened paper clip will do), and cut the end at an angle so it is sharp. Bend the other end into a rough loop or triangle, so the wire will stand up with the sharp point facing straight up. A little tape will help hold it onto the table, or a block of wood.

The armature (the part that spins) is made from the other piece of wire and the metal cap we saved when we took apart the fuse. Sharpen both ends of the wire by cutting the ends at a diagonal, like we did with the base wire. Bend the wire into an S shape. The pointed ends of the wire should point at 90 degrees from the center straight part of the wire.

Attach the metal cap to the center of the wire with tape. Place the cap onto the pointed end of the base wire, and bend the S shaped ends of the armature wire down, so it will balance easily on the sharp end of the base wire.

The armature should now spin freely if you tap it gently.

Connect a source of high voltage to the base wire using an alligator clip or a wire. The high voltage source can be the Van de Graaff generator, or just a couple square feet of aluminum foil pressed against the front of your television set, as we did in earlier projects.

As the high voltage is turned on, the armature will start to spin in the direction away from the sharp points. The Van de Graaff generator may need a good ground, or a person holding onto the ground wire. The television will give the motor a good kick every time it is turned on or off, and turning it on and off every second will get it spinning quite rapidly.

How does it do that?

The motor works by ionizing the air, and then pushing against the ionized air.

As we explained in the previous project, electric charges are concentrated by sharp points. The sharp points on the ends of the armature concentrate the charges so much that the air around the points becomes charged as well.