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The Honolulu Advertiser
Posted on: Sunday, August 5, 2001

Dr. Gadget's Science Machine
Electroscope exercise will open your eyes

By Joe Laszlo

A fun look at science working in the world around you, plus a cool gadget or experiment to test it out

Aloha! One of the gadgets I introduced last year was the electroscope. Since then, I've been going around to schools, showing students how to make it, and because of this I have discovered how to build an easier version. I'd like to share this with you because the electroscope is a fascinating gadget.

In case you didn't catch it the first time we talked about the electroscope, I'll explain: It is a gadget that helps you see static electricity at work. We make it with a pair of "vanes" that are suspended in a container. In the old version, we made the vanes with aluminum foil. But now I've found using either heavy-duty VCR recording tape or IBM data storage tape works better. Here's how to build it:

  1. Get a clear tennis-ball container. (You could also use a 20-ounce soda bottle and remove the label.)
  2. Make a hole in center of the container's lid with a pushpin. (If you're using a soda bottle, make a hole in the cap with a hammer and small nail; you may need a grownup's help for that).
  3. Get a large paper clip and bend it into the shape in Figure 1, which looks kind of like a sword.
  4. To make the vanes, cut a piece of heavy-duty VCR tape or IBM data tape about 5 1/2 inches long. Double it over with the dull side in. Using a pin, punch a small hole through the vanes an eighth of an inch below the fold (Figure 2).
  5. Now let's assemble the electroscope: From the outside of the cover or cap, insert the long end of the paper clip "sword" through the hole (Figure 3). Push it down so that it is sitting against the cap on both sides. Tape it down with masking tape so that the paper clip doesn't move.
  6. Thread the vanes onto the long end of the paper clip wire. Bend a small hook in the long end of the wire. Squeeze it tight to hold the vanes shut (Figure 4).
  7. Crumple a large piece of foil into a ball that is about 2 inches in diameter. Push the ball onto the "handle" of the "sword" where it protrudes from the lid. If it doesn't stay, secure it with some glue.
  8. Put the lid or cap back on the container, being careful not to crease your vanes (Figure 5).

Now you're ready to play with your electroscope! Blow up a balloon and tie it off. Charge it up with static electricity by rubbing it with a piece of silk or nylon, or on your hair. Test for a charge by holding it over your arm to see if the hairs stand up.

Bring the balloon near the foil ball on top of the electroscope. What do the vanes do? Do they split apart? Why are they doing this? For the answer, think about what happens if you bring the like poles of two magnets together: They repel each other. In the same way, the vanes of the electroscope both are getting the same charge. They both have electrons on them. The like charges on the vanes push them apart.

Let's try something different. Charge up the balloon again. Put a finger of your free hand onto the foil ball. Touch the charged balloon to the foil ball. Remove both hands at the same time. What happens? Now, bring the charged balloon near the foil ball. What happens to the vanes? Do the vanes close? Why is this happening?

Here's the explanation: When you brought the balloon close to the foil ball, some of the electrons in the system were shunted away to your finger. This left a net positive charge of protons on both of the vanes. Since they are the same charge, they push apart. But when you brought the balloon with the negative charge close to the system, the vanes closed because the electrons on the balloon neutralized the positive protons. Did you get all of that? Until next time ... a hui hou!

Dr. Gadget's Science Machine is written by Joe Laszlo, a retired science teacher and winner of a Presidential Award for Excellence in Science Teaching. Write: Dr. Gadget, c/o The Honolulu Advertiser, P.O. Box 3110, Honolulu, HI 96802, or fax 535-8170. You may send e-mail to ohana@honoluluadvertiser.com only if you're 13 or older.