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You can make a small, temporary incandescent-light demonstration by passing current through graphite pencil lead clamped between two bolt terminals beneath a glass jar. The graphite heats until it glows, but it can also smoke and combust, so assemble it with adult supervision, energize it only briefly, and let it cool before handling.
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What this project makes
The glowing element is graphite from a mechanical-pencil lead, not the wooden pencil casing or a finished commercial bulb. Graphite resists the flow of electricity; current through it produces heat, and at a high enough temperature it emits visible light. The Australian Science Teachers Association notes that the hot graphite can smoke as it combusts: its classroom activity.
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The two bolts pass through the jar lid as separated electrical terminals. Nuts hold them in place, and the upper nuts can clamp the graphite lead. A published bolt-based apparatus uses two 1/4-inch holes in a plastic lid, with a bolt and nuts at each hole: the apparatus description.
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Materials
- A clear glass jar with a lid, such as a mason jar.
- Two bolts and matching nuts to mount through the lid; the cited apparatus uses 1/4-inch holes.
- 0.5 mm graphite mechanical-pencil lead. IEEE Reach specifies this size and notes that it breaks easily, so have spare leads available: IEEE Reach’s activity.
- Two long insulated wires with alligator clips, or four shorter clip leads joined into two longer leads.
- Eight D-cell batteries connected in series, positive terminal to negative terminal. IEEE Reach describes eight 1.5 V cells as a 12 V classroom source.
- Electrical tape and a cardboard or paper-tube support if needed to keep the battery chain stable.
- Safety glasses or goggles, and a tray or stable nonflammable work surface.
Assemble and operate it safely
Do this only as a supervised, low-voltage classroom demonstration. Eight D cells can deliver substantial current. Wear eye protection before assembly, keep the battery leads from touching one another directly, and have an adult handle the final connection; the University of Northern Colorado also advises adult involvement for the final connection steps: UNC’s procedure.
- Mount the terminals. Make two holes in the plastic lid sized for the bolts. Insert one bolt through each hole and secure it with nuts so the bolts do not touch one another.
- Clamp the graphite. Position a piece of 0.5 mm lead between the upper nuts and tighten just enough to hold it without snapping it. Keep the lead intact and separated from the other bolt except through the intended filament path.
- Position the jar. Place the jar over the lid on the nonflammable work surface. Arrange the battery pack and clip leads so the leads can connect from the battery chain to the two bolts without crossing or touching each other.
- Check before powering. Confirm the graphite is unbroken, each clip has a firm connection to its terminal, the batteries are oriented in series, and there is no direct wire-to-wire short. Put on safety glasses.
- Complete the circuit briefly. Connect the battery chain so current passes through the bolts and graphite. Observe the lead glow, then disconnect the chain rather than leaving it powered.
- Wait before touching anything. The graphite remains hot after power is removed. The Australian Science Teachers Association worksheet says to wait five minutes before removing the jar or unclipping the lead.
Why the graphite glows—and why it does not last
When the circuit is complete, current flows through the graphite between the terminals. Electrical resistance converts some of that energy into heat; as the graphite becomes hot, it glows. The jar does not turn this into a durable household bulb: residual oxygen inside allows the hot graphite to smoke and combust. Commercial incandescent bulbs instead use a tungsten filament inside an evacuated or inert-gas-filled envelope, which helps the filament last longer. The National Gallery of Korea explains the role of the incandescent bulb’s envelope and filament: its overview.
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If it does not glow
- Check the battery chain: each battery must make contact with the next in positive-to-negative series.
- Check the clips and terminals: both sides of the circuit need a secure connection, and both clips must grip the graphite or its terminal path.
- Inspect the lead: a broken 0.5 mm lead will interrupt the circuit; replace it gently because this size is fragile.
- Check the circuit path: current should pass through the graphite, not bypass it through wires or touching bolts.
What affects the glow
Glow brightness and duration depend on several interacting factors: graphite length and diameter, battery count and available current, connection quality, and the oxygen remaining in the jar. A bolt-and-nichrome apparatus compared filament lengths of 10, 16, 22, and 28 cm, illustrating how filament geometry can be varied: the apparatus description. Those lengths are examples from that different filament setup, not established recommendations for this pencil-lead version.
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The cited sources do not report measured brightness, operating lifetime, temperature, or current for this exact jar-and-pencil-lead build, so those outcomes should not be treated as predictable specifications.
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