EXPERIMENT 3: Observing Radioactivity with an Electroscope

THINGS YOU NEED: A source of alpha rays (see inside front cover). Two water glasses (one about an inch taller than the other). An old 12-inch phonograph record. A piece of wool cloth. A four-inch length of 1/4-inch diameter wood dowel. A six-inch length of clean stiff wire. Aluminium foil. Foil from a chewing gum wrapper.

This experiment is similar to a late-19th-century method of observing the effects of radioactivity. It’s based on the fact that alpha particles can discharge an object that’s been charged with static electricity. The principle isn’t hard to understand.

When alpha particles strike atoms in the air, they “peel” electrons away from the atoms, leaving positively charged ions. These ions (and the negatively charged electrons) are then available to discharge the static electricity on any nearby object.

In this experiment, a simple electroscope indicates the presence of static electricity on the surface of a phonograph record. Make the electroscope as follows:

1. Twist one end of the wire around the center of thedowel. Leave a length of wire about three inches long protruding.

2. Bend a small L-shaped hook in the end of the wire.

3. Wrap and crumple some aluminum foil around themiddle of the dowel. The ball of foil you make should be about 3/4 inches in diameter; the foil must make good contact with the wire.

4. Soak the chewing gum wrapper in warm water to remove the paper liner. Cut a small strip about two inches long and 3/8 inches wide.

5. Fold the strip in half, then place it over the hook.

6. Carefully insert the entire assembly into the smaller glass; the strip should hang freely in the enter. Next, put the taller glass about four inches away from the electroscope. Rub one surface of the record with the wool cloth for several seconds to charge it with static electricity. Then place the record, charged side down, on the taller glass. The rim of the record must hang over the foil ball on the electroscope.