Materials for Lamp Filaments

The filament is an important part of incandescent lamp. The life of an incandescent lamp, depends on its filament. The filament material must have following merits-
  1. High melting point.
  2. Low vapour pressure.
  3. Free from oxidation in inert gas (i.e. argon, nitrogen etc.) medium at operating temperature.
  4. High resistivity.
  5. Low thermal coefficient of expansion.
  6. Low temperature coefficient of resistance.
  7. Should have high young modulus and tensile strength.
  8. Sufficient ductility so that can be drawn in the form of very thin wire.
  9. Ability to be converted in the shape of filament.
  10. High fatigue resistance against thermally induced fluctuating stresses.

List Materials used for Filament of Incandescent Lamp

Different type of material are used for making filament to f incandescent lamp. Some of these materials are listed below-
  • Carbon
  • Tantalum
  • Tungsten


Properties of Carbon

  1. Resistivity at 20oC : 1000-7000 µΩ -cm
  2. Temperature coefficient of resistanceat 20oC: -0.0002 to -0.0008 /oC
  3. Melting point: 3500oC
  4. Specific gravity: 2.1gm /cm3

Uses of Carbon

  1. Used for making pressure sensitive resistors which are used in automatic voltage regulators.
  2. Used for manufacturing the carbon brushes, which are used in DC machines. These carbon brushes improve the commutation as well as reduce the wear and tear.
  1. Commercial efficiency of carbon filament lamp is 4.5 lumens per watts or 3.5 watts per candle power
  2. To prevent the blackening of bulb, the working temperature is limited up to 1800oC.


Properties of Tantalum

  1. Resistivity at 20oC: 12.4 µΩ -cm
  2. Temperature coefficient of resistance at 20oC: 0.0036/oC
  3. Melting point: 2900oC
  4. Specific gravity: 16.6 gm /cm3
Note Commercial efficiency of Tantalum filament lamp is 3.6 watts per candle power. Due to low efficiency it is not much used these days.


Tungsten is produced by very complicated processes from rare ores or from tungstic acids. Some facts about tungsten are listed below-
  1. Very Hard
  2. Resistivity is twice to aluminum
  3. High tensile strength
  4. Can be drawn in the form of very thin wire
  5. Oxidize very quickly in the presence of oxygen
  6. Can be used up to 2000oC in the atmosphere of inert gases (Nitrogen, Argon etc.) without oxidation.

Properties of Tungsten

Properties of tungsten are listed below-
  1. Specific weight: 20 gm/cm3
  2. Resistivity at 20oC: 5.65µΩ -cm
  3. Temperature coefficient of resistanceat 20oC: 0.005 / oC
  4. Melting point: 3410oC
  5. Boiling point: 5900oC
  6. Thermal coefficient of expansion: 4.3 / K

Uses of Tungsten

  1. Used for making filaments of incandescent lamp.
  2. As electrode in X- ray tubes.
  3. The great hardness, high melting and boiling points make it suitable for use as electrical contact material in certain applications. It is having high resistance for destructive forces produces during operation of electrical contacts.
  1. The efficiency of tungsten filament lamp is about 12 lumen per watts.
  2. Operating temperature up to 2500oC.
  3. Do not have blackening effect on bulb like carbon filament have.
Due to above merits tungsten is preferred for making filament of incandescent lamp.

Compression of Filament Materials with respect to required Properties for Incandescent Lamp Filament

Sl No.PropertyCarbonTantalumTungsten
1Melting point (oC)350029003410
2Resistivity (µΩ-cm) at 20oC1000-700012.45.65
3Tensile strength (over 1650oC)lowestmediumHighest
4Blackening effectIt have blacking effect on bulbDoes not have blackening effect on BulbDoes not have blackening effect on Bulb
5Lumens efficiency (Lumens / watts)4.5 3.612
6Oxidation Resistance in inert gasLowestMedium Highest
7Thermal coefficient of expansion (/K)2 to 66.54.3
8Temperature coefficient of resistance (/oC)at 20oC-0.0002 to -0.00080.00360.005


Closely Related Articles Electrical Engineering MaterialsWhat is an Atom?Atomic Energy LevelsClassification of Engineering MaterialsClassification of Electrical Engineering MaterialsElectron Configuration of AtomPhysical Properties of MaterialsMechanical Properties of Engineering MaterialsRutherford Atomic ModelBohrs Atomic Model Chemical Properties of MaterialsEnergy Bands in CrystalsElectrical Properties of Engineering MaterialsFermi Dirac Distribution FunctionMagnetic Properties of Engineering MaterialsThermionic EmissionSelection of Material for Engineering ApplicationBases of existence of properties in materialsQuantum NumbersLow Resistivity or High Conductivity of Conducting MaterialHigh Resistivity or Low Conductivity Conducting MaterialFactors Effecting the Resistivity of Electrical MaterialsMaterials used for Heating ElementsMaterials used for Precious WorksMaterials Used for Transmission Line ConductorElectrical Stranded ConductorsElectrical ConductorMaterials used for RheostatsClassification of Electrical Conducting MaterialApplications of Carbon Materials in Electrical EngineeringSelection of Materials Used for Electrical ContactsBimetalsIonic PolarizationPiezoelectricityDielectric MaterialsMechanism of PolarizationDielectric Material as an Electric Field MediumOrientational PolarizationElectric Arc FurnaceThermal Conductivity of MetalsFree Electron Theory of MetalsMagnetostrictionAntiferroelectricityFerroelectric MaterialsElectronic PolarizationFerromagnetic MaterialsMore Related Articles Thomson Plum Pudding Model (1911)New Articles Principle of Water Content Test of Insulating OilCollecting Oil Sample from Oil Immersed Electrical EquipmentCauses of Insulating Oil DeteriorationAcidity Test of Transformer Insulating OilMagnetic Flux