What is the wire that gives off light in a light bulb?

The wire that gives off light in a light bulb is the filament. In a traditional incandescent bulb, electric current passes through a very thin tungsten wire, heating it until it glows brightly enough to produce visible light.

Tungsten is used because it has an exceptionally high melting point—about 3,695 kelvins—and can remain solid at the roughly 2,000–3,000 K temperatures used in incandescent lamps. The filament is enclosed in a glass bulb filled with a vacuum or inert gas, which prevents hot tungsten from rapidly reacting with oxygen.

Earlier bulbs used carbon filaments, including carbonized paper and bamboo. Tungsten filaments became important in the early 20th century because they were more efficient and longer-lasting. A common mix-up is calling the entire bulb the filament: the bulb is the glass enclosure, while the filament is the glowing wire inside.

Incandescent filaments eventually become thinner as tungsten evaporates. A tiny weak spot then breaks, which is why a conventional bulb suddenly goes dark. LEDs produce light differently and do not use a glowing filament.