A Carbon Arc Experiment That Can Be Done in the Classroom
As long as proper safety precautions are followed, a simple experiment can be set up to demonstrate the carbon arc.
Materials
Materials needed are goggles or dark film suitable for viewing solar eclipses for everyone who will be looking at the arc, two of the large lead pencils used in the primary grades, a 12 volt car battery fully charged, a cup of baking soda, a sponge a pail of water, two terminal clamps for the battery, and two 6 foot pieces of heavy (12 gauge or higher) insulated wire. The water, baking soda, and sponge are for quick cleanup and neutralization of any spilled acid from the battery. It is best to choose a maintenance free battery to minimize chance for spills.
Setup
- Strip about two inches of insulation from each end of both pieces of wire.
- Remove about an inch of wood from one side of both pencils down near the eraser end. The idea is to expose the graphite core so the wires can be connected but leave enough support for the core, as it is quite brittle.
- Wrap the 2 inch bare wire on one end of a wire tightly around the exposed graphite core of one of the pencils. Hook the other end of the wire to the battery terminal clamp. Hook the other pencil, wire, and clamp the same way. Make sure the connections are tight and that plenty of wire comes into contact with plenty of graphite.
- Hook up both battery clamps to the battery.
Procedure
- Put on safety glasses or eye protection and have everyone else do so.
- Bring both pencil tips together and carefully pull them about an eighth of a centimeter apart. The trick is to strike a brilliant arc. Vary the distance until you get maximum brilliance. Do this for only a few seconds, then stop for a while for the pencils to cool off.
- Experiment with the sharpness of the pencils, shape of the tips, etc. What happens after you draw a few arcs? Does the tip of the pencil erode away? If so, why do you think it does?
Safety Precautions
- NEVER look at the brilliant arc without proper eye protection.
- Welding goggles are suitable for the one demonstrating the experiment..
- Glasses or filters suitable for observing eclipses may be used by the students, as may welding goggles.
- Regular sunglasses will NOT protect the wearer from sunburning of the retina with ultraviolet light produced in this experiment
- AVOID spilling the battery electrolyte or tipping the battery.
- Sponge off the top of the battery and the area around the terminals with baking soda and water solution before you start and if you suspect that acid has been spilt.
- AVOID touching the terminals too much. They contain lead.
- The pencils can get quite hot. Be careful to not tough the tips with your finger until they cool down.
- If you have touched the battery, wash your hands before touching face, eyes, or mouth.
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