Anyone who has turned a volume knob on a radio, stereo or TV has used a potentiometer. These electrical components, also called variable resistors or "pots," were much more common before digital circuits took over, but are still used in many consumer electronic products. Most pots are small, round cylinders with a central rotating shaft and three electrical connections. The purpose of the device is to vary the amount of voltage or current that passes through as the knob turns from one side to the other. Try this simple experiment that demonstrates how a potentiometer works.
Instructions
1. Scrape the wood from one side of a No. 2 pencil with a pocket knife until the lead is exposed evenly across the length of the pencil.
2. Disassemble a flashlight, and remove one battery as well as the light bulb. Observe how the flashlight is assembled so it can be put back together when the experiment is complete.
3. Cut three lengths of wire, each about 6 inches long. Strip about a half-inch of insulation from each end of the wires.
4. Tape one wire to each end of the battery and the remaining wire to the outer contact of the light bulb. Note: most bulbs have a metal base with a small contact at the bottom center of the base. Wrap the wire around the metal base, then tape it so the other contact is still accessible.
5. Wrap the wires coming from the battery around each end of the pencil lead, one toward the eraser and the other toward the end that would be sharpened. Make sure the stripped wire touches the exposed pencil lead.
6. Wrap the other end of the wire attached to the light bulb to the eraser end of the pencil, again making contact with the lead.
7. Hold the bulb over the far end of the pencil so the center contact touches the lead. The bulb should light. As the bulb is moved toward the eraser side of the pencil, the bulb will slowly dim.
The pencil lead contains a mixture of graphite, a form of carbon, and clay. Graphite is an electrical resistor. It conducts electricity, but absorbs some of the energy along the way. As the bulb moves away from the wire, less electricity gets to the bulb, so the bulb dims.
Potentiometers found in radios and other devices use the same principle. The two outer contacts are each connected to an arc of resistive material, and the center contact connects to a brush that moves as the shaft rotates. As the brush moves away from the end, the resistance increases.
Tags: light bulb, pencil lead, away from, center contact, from side, metal base, moves away