Electro-Scope It Out!

What You Need:

  • Small glass jar
  • Wire coat hanger
  • Aluminum foil
  • Cardboard
  • Plastic comb
  • Balloon
  • Pencil
  • Cellophane tape or Scotch tape
  • Rubber cement
  • Electrical tape
  • Wire clippers
  • Pliers

What You Do:

  1. Use the wire clippers to cut off a straight piece of wire from the hanger. Use the pliers to bend a section at one end into an L shape.
  2. Turn the jar upside down onto the cardboard, and trace a circle around the opening.
  3. Cut out the circle and punch a small hole in its center with a pencil.
  4. Carefully push the wire through the hole about 1 inch (2.5 cm), straight end first.
  5. With as little cellophane tape as possible, attach a strip of aluminum foil to the bent end of the wire so that the strip hangs down in two equal halves.
  6. Rubber cement the cardboard circle to the top of the jar, with the bent end of the wire holding the aluminum strip pointing down.
  7. Place electrical tape around the edges where the circle touches the rim of the jar, and place a thin band of cement around the wire where it punches through the cardboard.
  8. After the rubber cement dries, crumple the aluminum foil into a tight ball, and carefully push it onto the top of the wire.
  9. Rub the plastic comb or balloon against your hair or clothing (wool works best), and hold either one close to the aluminum foil ball.

If the air is dry enough, the ends of the Mylar strip fly apart when the comb or balloon touches the ball.

What's Going On?

An electroscope shows the attraction and repulsion of electrical charges. In all electrical activity, like charges repel and opposite charges attract. When you rub the comb, friction causes a positive charge to build up in the plastic. When you hold the positively-charged comb near the aluminum foil ball, the comb attracts negative charges which move up through the wire so that only positive charges remain in the Mylar strip. Since both ends of the strip now have the same charge, the ends of the strip fly apart!

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