Glass bulbThe glass envelope that seals in the filament and the gas. On an ordinary bulb it can reach 200 to 260 °C.
FilamentA fine coil of tungsten wire. The current heats it until it glows white-hot and gives off light.
Inert gasThe low-pressure gas that fills the bulb. Examples are argon, nitrogen, krypton and xenon. If air got in, it would react with the hot tungsten.
CapThe metal base that screws into the socket. It carries one side of the circuit. It is also called the sleeve or base.
Inside the glass: 3 3
Contact wiresTwo wires that carry the current to each end of the filament. They run up through the stem.
Support wiresThin wires set in the stem that hold the filament in place. No current flows through them.
StemThe glass mount rising from the neck. The wires are sealed into it.
The base: 2 2
InsulationA glassy band between the cap and the electrical contact. It is called vitrite.
Electrical contactThe metal tip at the bottom. It touches the center of the socket and carries the other side of the circuit.
How this list was compiled: Read 2026-09-27. Wikipedia, Incandescent light bulb, https://en.wikipedia.org/wiki/Incandescent_light_bulb : its labelled bulb names the glass bulb, low-pressure inert gas (argon, nitrogen, krypton, xenon), tungsten filament, contact wires going into and out of the stem, support wires (one end in the stem, conducting no current), the stem (glass mount), the cap (sleeve), the insulation (vitrite) and the electrical contact; the glass of a general service lamp can reach 200 to 260 °C; a leaking envelope lets the hot tungsten react with air. Licence: own work
The drawing: Drawn for this site by script, 2026-09-27. Own work, nothing traced. Licence: own work
How it works
These are the parts of a lightbulb on a clear incandescent bulb, the kind with a glowing filament. Start with the 4 main parts, then the inside, then the base.
Read the full guide
Inside the glass
The filament is a fine coil of tungsten. Two contact wires carry the current up through the glass stem to its ends, and thin support wires hold it up. The bulb is filled with an inert gas at low pressure.
The base
The threaded cap screws into the socket and is one side of the circuit. The electrical contact at the tip is the other, with a band of insulation between them.
The current goes in at the tip, up one contact wire, through the filament, and down the other wire to the cap.
Questions and answers
What is the filament in a light bulb made of?
Tungsten. The current heats the fine coil until it glows white-hot and gives off light.
What gas is inside a light bulb?
An inert gas at low pressure, such as argon, nitrogen, krypton or xenon. It keeps air, which would react with the hot tungsten, away from the filament.
What is the metal base of a light bulb called?
The cap, also called the base or sleeve. Its thread screws into the socket, and the contact at its tip completes the circuit.