In a light bulb the filament is a thin wire that glows when a current flows throgh it. The first light bulbs used a carbon thread as a filament. Today tungsten filaments is the most used due to its high melting point. Most light bulbs contain some inert gas since the tungsten filament vaporizes quickly in vaccum. the most used gas is argon and nitrogen. But in some lamps the argon is replaced with another gas , like in the halogen lamps and krypton bulbs.

Filament also refers to a wire structure found in tubes. When the it is heated by the proper voltage and current, the filament emits electrons. These electrons are attracted to the positive element of the tube, the plate (or anode). In multi-element tubes, the electron stream may pass through grids or beam-forming elements on its way to the plate. In this type of tube, the filament forms part of the circuit and is considered to be negative.

In tubes that have a separate cathode, the filament is placed inside the heater (a metal cylinder) and is used to bring the heater to operating temperature. The filament does not usually form part of the circuit in this type of tube.

Fil"a*ment (?), n. [F. filament, fr. L. filum thread. See File a row.]

A thread or threadlike object or appendage; a fiber; esp. Bot., the threadlike part of the stamen supporting the anther.

 

© Webster 1913.