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Monday, 29 July 2019

Materials used for Lamp Filament

Usually carbon, tantalum and tungsten are used for manufacture of filament. Besides the properties of carbon mentioned above, its commercial efficiency of carbon filament lamp is 4.5 lumens per watt or 3.5 watts per candle power. To prevent the blackening of the bulb, the working temperature is only 1800°C. Tantalum is a material whose resistivity is 12.4 micro ohm-cm. Its temperature co-efficient of resistance is 0.0036 per degree and its melting point is 2900°C. Its specific gravity is 16.6. Its efficiency is about 1.6 watts per candle power. Due to its low efficiency it is not much in use. Tungsten has a high melting point and besides the properties mentioned previously, the efficiency of tungsten filament at a working temperature of 2700°C in an evacuated bulb is 12 lumens per watt.

The steps followed for the preparation of tungsten filament is :

(a) Chemically pure tungsten oxide is reduced at red heat in an atmosphere of hydrogen to metallic tungsten in the form of grey powder.

(b) The powder is pressed in steel moulds into small bars under hydraulic pressure.

(c) The bars are then heated in the furnace upto 1100°C in the presence of hydrogen whereby the articles sinter together. This imparts mechanical strength to the bars.

(d) The mechanical strength is improved further by heating (electrically) the bars almost to melting point.

(e) The bars are then hammered or rolled at red heat to make them ductile and finally drawn into filaments.
To improve the efficiency of the bulb, it is filled with an inert gas, orgon with a small percentage of nitrogen. To minimise the convection currents produced by molecules of the gas in bulbs, the filament is wound into close spiral and suspended horizontally in the form of a circular arc. In high wattage bulbs necks are provided. This is done to avoid blackening of the bulbs as the convection currents carrying particles from the filaments blacken the neck only without impairing the candle power in the direction below the horizontal. The efficiency of gas filled coiled coil tungsten filament lamps is about 25 lumens per watt or 0.6 watts per candle power and thus these lamps are called half watt lamp. Figure shows the straight and coil form of tungsten filament.

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