"Dimmer and Dimmer" Created by Doug Spicher
Elkridge Landing Middle School

Elkridge, Maryland

This lesson is designed for 8th grade and should take one 45 minute period.




Dimmer and Dimmer and Dimmer and Dimmer


Many of us have used a dimmer switch. If we have not, then we probably have turned a knob or pressed a button on a radio or television to control the volume. If so, then you have used a rheostat. A Rheostat is used to change the resistance of an electric circuit. In other words, it is used to allow fewer electrons to flow through a circuit while still allowing the consumer of power, usually an electric light, to stay on!

Today, you will model a rheostat to see how they work.



  1. Make sure you wash your hands completely before you are done…and do not touch your eyes until you have cleaned your hands!

  2. Attach alligator clips to the ends of each wire.

  3. Attach one clip to the positive battery pole and then to one side of the socket (connected)

  4. Attach another clip to the other end of the socket and then to the graphite.

  5. Attach the remaining clip to the negative pole of the battery.

  6. Here is where things get tricky. Move the alligator clips further apart and closer together. Notice what the light does as you change position of the clips.

  7. Disconnect all wires and alligator clips.

  8. Return the graphite to your teacher.

  9. Wash your hands!


  1. What happened to the brightness of the bulb as the clips got closer together?

  2. What happened to the brightness of the bulb as the clips got further apart?

Questions and Conclusions

  1. Why did to bulb get brighter when the clips were closer together?

  2. Which part of the rheostat that you have made controls the flow of the current?

  3. A good conductor, like copper, was not used in this lab as it is not used in real rheostats. Why wouldn’t copper work in a rheostat?

  4. List at least three ways or three items that use a rheostat.