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Materials Used in Lamp Filaments
The filament is the most important part of an incandescent lamp. The life of the incandescent lamp depends upon its filament. The ideal material used for the filament of the incandescent lamps is the one which has the following properties −
It should have high melting point.
Low temperature coefficient.
High resistivity.
Low vapor pressure.
It should be ductile.
It should have sufficient mechanical strength to withstand the vibrations during the operation.
Low thermal coefficient of expansion.
Materials Used in Lamp Filament
Different types of materials are used for making the filament of an incandescent lamp, some of these materials along with their properties are described below −
Carbon
Carbon may be used for making the filament of the incandescent lamps. The carbon has the following properties which make it suitable materials for the filament of the lamp −
Resistivity ($\rho$ ) - The resistivity of carbon ranges from 1000 $\mu \Omega$-cm to 7000 $\mu \Omega$-cm.
Temperature coefficient of resistance $(\alpha)$ - For carbon, the temperature coefficient of resistance varies from -0.0002 to -0.0008.
Melting point - Carbon has the melting point equal to 3500 °C.
Density - It has density between 1.7 and 3.5.
Note −
In order to prevent the blackening of the incandescent bulb, the working temperature is limited to 1800 °C.
The commercial efficiency of the incandescent lamp using carbon filament is about 4.5 lumens per Watt.
Tantalum
Tantalum may also be used for making the filament of the incandescent lamps. The properties of tantalum are as follows −
Resistivity $(\rho)$ - Resistivity of the tantalum is about 12.4 $\mu \Omega$-cm.
Temperature coefficient of resistance $(\alpha)$ - Tantalum has temperature coefficient of resistance equal to 0.0036.
Melting point - The melting point for the tantalum is equal to 2996 °C.
Density - Tantalum has density equal to 16.6.
Note −
The tantalum is not used much now-a-days, because the incandescent lamp having tantalum filament has very low efficiency such as 2 lumens per Watt.
Tungsten
Tungsten is the most widely used metal for making the filament of the incandescent lamps. The properties of the tungsten metal are as follows −
Resistivity $(\rho)$ - Resistivity of the tungsten is equal to 5.6 $\mu \Omega$-cm.
Temperature coefficient of resistance $(\alpha)$ - Tungsten has temperature coefficient of resistance equal to 0.0045.
Melting point - The melting point for the tungsten is equal to 3400 °C.
Density - Density of the tungsten metal is 19.3.
Note −
An incandescent lamp having tungsten filament and working at 2000 °C has high efficiency about 18 lumens per watt.
To improve the efficiency of a tungsten filament lamp, the bulb is filled with an inert gas such as argon with a small percentage of nitrogen.
The efficiency of a gas filled coiled coil tungsten filament is about 30 lumens per Watt and it is because of the high operating temperature of 2500 °C.
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