In this project we will build a device that can generate 12,000 volts from an empty soda can and a rubber band.
The device is called a Van de Graaff generator. Science museums and research facilities have large versions that generate potentials in the hundreds of thousands of volts. Ours is more modest, but is still capable of drawing 1/2 inch sparks from the soda can to my finger. The spark is harmless, and similar to the jolt you get from a doorknob after scuffing your feet on the carpet.
To build the toy, you need:
• An empty soda can
• A small nail
• A rubber band, 1/4 inch by 3 or 4 inches
• A 5x20 millimeter GMA-Type electrical fuse (such as Radio Shack #270-1062)
• A small DC motor (such as Radio Shack #273–223)
• A battery clip (Radio Shack #270–324)
• A battery holder (Radio Shack #270–382)
• A styrofoam cup (a paper cup will also work)
• A hot glue gun (or regular glue if you don't mind waiting)
• Two 6 inch long stranded electrical wires (such as from an extension cord)
• Two pieces of 3/4 inch PVC plumbing pipe, each about 2 or 3 inches long
• One 3/4 inch PVC coupler
• One 3/4 inch PVC T connector
• Some electrical tape
• A block of wood
That sounds like a lot of stuff, but take a look at the step-by-step photos below, and you will find that the whole project can easily be put together in an evening, once all the parts have been collected.
We'll start at the bottom, and work our way up.
The first thing to do is to cut a 2 to 3 inch long piece of 3/4 inch PVC pipe, and glue that to the wooden base. This piece will hold the generator up, and allow us to remove it to more easily replace the rubber band, or make adjustments.
The PVC "T" connector will hold the small motor. The motor fits too loosely by itself, so we wrap paper or tape around it to make a snug fit. The shaft of the motor can be left bare, but the generator will work a little better if it is made fatter by wrapping tape around it, or (better) putting a plastic rod with a hole in the center onto the shaft to act as a pulley for the rubber band.
Next, we drill a small hole in the side of the PVC "T" connector, just under the makeshift pulley on the motor. This hole will be used to hold the lower "brush", which is simply a bit of stranded wire frayed at the end, that is almost touching the rubber band on the pulley.
As the photo shows, the stranded wire is held in place with some electrical tape, or some other tape or glue.
The rubber band is now placed on the pulley, and allowed to hang out the top of the "T" connector.
Next, cut another 3 or 4 inch piece of 3/4 inch PVC plumbing pipe. This will go into the top of the "T" connector, with the rubber band going up through it. Use the small nail to hold the rubber band in place, as in the photo below. The length of the PVC pipe should be just enough to fit the rubber band. The rubber band should not be stretched too tightly, since the resulting friction would prevent the motor from turning properly, and increase wear on the parts.
Cut the styrofoam cup about an inch from the bottom, and carefully cut a 3/4 inch diameter hole in the center of the bottom of the cup. This hole should fit snugly onto the 3/4 inch PVC pipe.
Now drill three holes near the top of the PVC union coupling. Two of these holes need to be diametrically opposite one another, since they will hold the small nail which will act as an axle for the rubber band. The third hole is between the other two, and it will hold the top "brush", which, like the bottom brush, will almost touch the rubber band.
The top brush is taped to the PVC union coupler, and the coupler is placed on the 3/4 inch pipe, above the styrofoam cup collar. The rubber band is threaded through the coupler, and held in place with the small nail, as before.
Bare the top brush (so it has no insulation) and twist it to keep the individual wires from coming apart. You can solder the free end if you like, but it is not necessary.
The free end of the top brush will be curled up inside the empty soda can when we are done, and thus electrically connect the soda can to the top brush.
We need a small glass tube to act as both a low-friction top pulley, and as a "triboelectric" complement to the rubber band, to generate static electricity by rubbing. Glass is one of the best materials to rub against rubber to create electricity.
We get the tube by taking apart a small electrical fuse. The metal ends of the fuse come off easily if heated with a soldering iron or a match. The solder inside them drips out when they come off, so be careful. The glass, the metal cap, and the molten solder are all quite hot, and will blister the skin if you touch them before they cool.
Save the metal caps — we will use them in a future project!
The resulting glass tube has nice straight, even edges, which are "fire polished" for you, so there is no sharp glass, and no uneven edges to catch on the PVC and break the glass.
The next step is a little tricky. The
