Make a Lightbulb

An incandescent light bulb throws off light because it releases electromagnetic radiation when the tiny wires inside are heated. You, too, can produce visible light by building a simple incandescent lamp with a wire filament in a jar, and a source of electricity, and some copper wire.

Materials:

• One medium jar with a lid
• 12 inches of copper lamp wire, or speaker wire, or the electric cord from a broken appliance (e.g. iron, coffee maker, clock)
• A roll of twisted iron, or steel picture-hanging wire
• A nail and hammer
• One 6-volt lantern battery
• Scissors or pliers

How to Do It

1. Wash and dry an empty mayonnaise or pickle jar. (Note: a jelly jar may be too small.)
2. Use the hammer and nail to punch two holes about an inch apart in the lid of the jar.
3. Use scissors and cut or tear the electrical cord in half so you have two lengths of copper wire. (Note: Speaker wire cannot be split – use two 12-inch pieces.)
4. Remove two inches of the plastic cover from the four ends of the copper wires.
5. Thread the two wires down through the two holes in the lid of the jar to a length of 4 inches.
6. To make a filament, cut a two-inch piece of iron wire (the filament) from the roll and attach it to the two copper wires hanging down from the lid by twisting the iron wire around each end of the copper wire. The iron wire stretches across the space between the two copper wires. Now you have a filament.
7. Place the lid on the jar and attach the other two ends of copper wire outside the jar to the 6-volt lantern battery by twisting the copper wire around the posts of the battery. The copper wire must be in contact with the metal posts.
8. Watch the iron wire get red hot and glow with an orange light. (Note: the iron wire will be too hot to touch for several minutes.)

How It Works

Your experiment is similar to the way Thomas Edison invented the incandescent light bulb. He used platinum wires in his experiments and finally settled on a carbon filament. Today’s incandescent bulbs have a tungsten alloy filament in a vacuum, so they last a long time.

When electricity flows from the battery through the copper wire, it meets resistance from the iron wire. The iron wire gets red hot and finally glows with an orange light. The iron wire breaks because it gets so hot; it oxidizes (burns) in the air in the jar.

The light is produced because electrons are trying to get through the iron wire; some moving electrons escape as photons. When the iron wire gets hot, it releases some of its thermal vibration energy as photons. Incandescence is the emission of light by a solid that has been heated.

Variations

Use a doubly thick iron wire (the filament) and notice how long the light lasts before the wire breaks.

Use a longer piece of iron wire and see if there is a difference in the brightness of the light.

Use a small 9-volt battery. Tape the copper wire to the two posts. Observe the change.

Use a variety of batteries (C or D size) connected in series (+ positive to - negative).

Prevent the iron wire from burning by filling the jar with carbon dioxide. Use one tablespoon of baking soda and two tablespoons of vinegar. Let the reaction bubble and then put the lid on the jar and connect the battery!

You could skip using a jar and just make the wire connections to the battery, but the jar keeps you from burning yourself. It also mimics a glass light bulb.