Electric light bulb, coloured X-ray. This consists of a glass bulb that seals a metal filament (upper centre) under an inert gas, usually an nitrogen- argon mixture. The screw fitting (lower centre) is where the bulb connects to an electricity supply, allowing an electric current to flow through the filament. This filament is made of a high-melting point metal (such as tungsten). Since the filament is thin, it heats up as the current passes through it, and glows to light up the surroundings through the transparent bulb. Although protected from the oxygen in the atmosphere, the metal filament still evaporates and eventually breaks. The bulb then needs to be replaced.