1. A flashlight bulb and socket
  2. Two "D" cell batteries or a dry cell lantern battery
  3. About 16 inches of wire
  4. A piece of wire about 2 inches long
  5. A long, long spring -- the spring from a inside w roll up window will work great. You'll need to break open the wooden section that the shade rolls up on to get to the spring. Ask an adult to help you get this spring out.
  6. Pair of wire clippers

Connect the two batteries so the positive pole connects to the negative pole of the other battery.

Cut the 16 inch wire in half and attach each piece to the open end of the joined batteries.

Connect the free end of one wire to one terminal of the light socket.

Connect the other free wire to one end of the spring.

Connect the two-inch long wire to the other terminal of the light socket.

Connect the wire coming off the terminal to the end of the end where the other wire is. Watch how brightly the bulb glows.

Now slowly move the short wire down the length of the spring. What happens?

As you go further away from the end where the wire is attached to the spring, the light will get dimmer and dimmer. The steel wire in the spring is not a very good conductor of electricity. The more wire the electricity has to pass through the more resistance there is and the less electricity. What you've made is called a rheostat. This is a device to vary the amount of a current passing through it to complete a circuit.


© 2006 California Energy Commission. All rights reserved.