Graphite Variable Resistor

The graphite inside of pencils acts as a resistor due to its unique structure of carbon atoms arranged in hexagonal rings. Each carbon atom in this structure can form 3 covalent bonds with the other atoms, leaving one electron free. These free or delocalized electrons move, conducting electricity. However, graphite is not a perfect conductor of electricity because van der Waals forces (weak electrostatic forces that attract neutral molecules to one another) hold the graphite layers together and make it harder for the delocalized electrons to move through them.

One thing to note is that the graphite inside pencils is not pure-- often, it is mixed with clay. To get a purer graphite resistor, use softer pencils like 6B or 8B, which allow the electricity to flow through the resistor better. The longer you make the paper, the higher the resistance as well, as the electricity has to move through more 'layers' of graphite.

1x LED

2x 1.5 volt batteries and battery pack (with aligator clips)

1x alligator clips

1x pencil (4B or softer pencils work best)-- I used an 8B pencil for this

Paper

Scissors

Cut a small rectangle of paper. Color it in completely using the pencil, making sure no white areas are remaining and the graphite layer is dark and shiny.

Connect the positive end of the battery to one end of the graphite strip.

Connect the other end of the paper to an alligator clip. On the other end of the alligator clip, attach the positive (longer) leg of the LED.

Finish the circuit by connecting the negative (shorter) end of the LED to the negative end of the battery.

Experiment with the resistance by moving the alligator clips closer and farther apart on the graphite strip. The closer they are, the less the resistance and the brighter the LED glows.