Drill a 1/4-inch hole in the center of the jar's lid. Use the drill to enlarge the hole if necessary so that you can slide the soda straw into it. Cut a 4-inch length of straw and insert it halfway into the hole so that you have 2 inches of the straw on either side of the lid's surface. Glue the straw into place such that it will stand upright when the lid is on the jar.
Bend the wire at the halfway point to form a 90-degree angle. With the pliers parallel to one leg of the angle, secure the other leg right next to where you made the bend. Start wrapping this second leg around to form a spiral disc with the tip of the pliers at the center. In the end you should have what looks like a copper flower with a 6-inch stem.
Insert the 6-inch copper stem into the soda straw such that the spiral disc can rest on the top of the straw. Two inches of the wire should protrude from the bottom end of the straw. With the pliers, bend the length of wire that is protruding into a hook shape.
Fold a piece of aluminum foil onto itself so that you can cut out two identical patterns at the same time. With the scissors, cut out an oblong egg shape 1 1/2 inches in length that almost looks like a tear drop with one end narrow and the other wide. Since you doubled the aluminum foil over, you should end up with two identical egg shapes.
Line up the two egg-shaped pieces of aluminum foil so that you can make a hole in the same place through them both. Using a brad nail, puncture a hole near the edge at the narrow end of your egg-shaped pieces.
Slide the copper hook through the hole and flatten out any crinkles that you may have made in the aluminum foil. Screw the lid on the jar, and your electroscope is ready. The two pieces of aluminum foil should be dangling from the hook face to face with the wide end of the egg shape toward the bottom.
Rub a balloon against your hair and position it near the spiral coil of your electroscope. You should see the two pieces of aluminum foil separate kind of like a butterfly spreading its wings. This indicates that you have a build up of electrons on the balloon's surface.