Testing Resistance
Part I: The purpose of this inquiry is to test the effect of varying length of carbon in a pencil on an electric circuit.
1. Prepare a pencil lead. Although this is called "lead", it is actually made of compressed carbon atoms. Carbon is a basic element found in nature that conducts electricity. You can either find a regular pencil and then sand, notch, or file divots at various intervals as shown in the middle diagram, cut the pencil in half by sanding or filing it away as it shows at the bottom, or find a replacement lead that will not easily break like that shown at the top.
2. The next step is to prepare a circuit. You will need the following:
A battery of 3 Volts (2 AA, 2 C, or 2 D cells in series)
A bulb from a string of indoor/outdoor miniature lights
Connectors, wire and clips
3. Connect your wires as shown in the following pictorial diagram:
Make sure that the connections are tight and that the clips or wires make good contact to the carbon. Confirm that the bulb will illuminate when connected to various points on the pencil lead.
4. Conduct a test of the relative brightness of the bulb as it is connected with progressively longer lengths of carbon. Watch the following video as a model:
5. Record data in a data table by reporting the length of carbon tested, the hardness of the pencil (1H, 2H, 4H etc) and the relative brightness of the bulb. Brightness can be expressed as "very bright, bright, less bright, dim, very dim, off." Challenge students to find other ways to quantify brightness such as how far they are able to see a glow.
For Example:
2 H Pencil Resistance test
6. Try other pencil leads as a comparison.
Part 2: If you want to see how resistors are made, you can create your own resistor by using a soft pencil to draw carbon on an index card. Remember that "lead" is really carbon. Lead was used in early times, but carbon has long replaced this toxic element in pencils.
We used a soft art pencil to form the resistor in this example.
Note how the rate of clicking slows as the length of the carbon increases. Listen to what happens if the uninsulated ends of the clips are only in contact with the fingers. The human body contains water and salt--another example of a resistance.
The value of the resistor is dependent on the length and density of the carbon. By decreasing the amount of carbon on the paper one can make larger resistances like the 1,000,000 ohm resistor. If one adds a great deal of carbon with more and more layers of penciling, then a lower resistance can be made.