Science Buddies: "Ask an Expert"

Hi there. We have finished a science fair project based on the Pencil Resistor project here on Science Buddies. My son is in 5th grade. He used a multimeter to calculate the resistance of three different lengths of pencils in a circuit. HOWEVER, the results of the calculations have us stumped. Why would our 15.5 cm pencil have a lower resistance value than our 8 cm. pencil? HELP!

Hello and welcome to the forums!

I don't know exactly what you've done in your experiment, but I did read the write up and can offer some help.

First, how did you measure / calculate the resistance? If you used the OHMs setting, are you sure about the ranges used? If you measured voltage across and current through the pencil, you may have introduced error with the multimeter. Please explain.

Also, the resistance value you measure will be dependent on how well you can connect to the graphite in the pencil. Have you done multiple trials? Are both ends of the pencil scraped clean of wood (or plastic)?

Finally, are the pencils all the same, just different lengths?

Please let us know more about what you did and we'll try to help.
Keith