An electroscope is an instrument used by scientists to measure the relative strength of an electric charge. A simplified version of an electroscope can be made easily and can be used to study and explore static electric charges.
Clear plastic cup
Modeling clay or plastic tape
What To Do
Make a small hole in the bottom of the plastic cup through which the paperclip will later be inserted. *Note: Try using a hot glue gun without any glue to melt a small hole, or heat the paperclip and push the end of it through the cup. Cut two strips of foil that measure roughly ¼ inch by 1 ½ inch. Use the end of a paper clip to punch small holes in the one end of each foil strip. Unfold a paperclip so that it looks like a long J, and hang the foil strips, called leaves, on the curved end of the J. Holding the cup upside-down, insert the straight part of the J paper clip through the hole in the cup, so the leaves hang inside the upside down cup without touching the table or desk top. Secure the paperclip using molding clay or plastic tape. Roll some aluminum foil into a ball and place the ball on the top of the paperclip that is sticking out from the cup. The electroscope is complete and ready for use. Charge a balloon by rubbing it with a piece of wool or fur, or by rubbing it in your hair. Slowly bring the charged balloon near the foil ball on the electroscope and watch how the leaves react. Move the balloon away and observe the leaves of the electroscope.
1. What made the leaves move? How? Why?
2. Does anything else coming near the foil ball on the electroscope have the same effect? What?
3. Do the leaves move more or less if the balloon is more charged? Why?
The leaves of the electroscope moved away from each other because they both acquired a negative charge and repelled each other. The negatively charged balloon coming near the foil repelled some of the electrons in the foil. Those electrons travel down the paper clip to the leaves, giving each of them extra electrons and thus a negative charge. Like charges repel, so the leaves moved away from each other.
"Awesome Experiments in Electricity and Magnetism." Michael DiSpezio, Sterling Publishing Co.: New York, 1998, p. 62-63.
© S. Olesik, WOW Project, Ohio State University, 2001.