Carbon wire filament incandescent light bulb

This carbon filament light bulb has two selectable filaments, for high and low power. The switch is located in the neck, and twisting the bulb switches between the two filaments. Carbon filament lamps were the first incandescent light bulbs, in which a filament emits light when heated by an electric current. The filament must be kept in a vacuum to prevent it from burning up, and this is why the filament is enclosed in a glass bulb.