The filament of this lamp is heated up to the incandescent stage of heat. So these types of lamps are called as incandescent lamps.
There are two types.
1. Vacuum type lamp and
2. Gas filled type lamp
In this type of lamp, the sphere shaped glass cover is used. The glass stem is fixed in the centre of the lamp. This stem supports wires in holding the filament. The top of the lamp is sealed. Pins are used for holding the lamp in the holder.
In this lamp, the air is evacuated to protect the filament from burning by oxygen mixed in the air.
When the filament is heated, due to the current that passes through the high resistance filament, the moving electron creates friction. So the heat is generated in the filament. When the heat raises up to the incandescent stage, the light is emitted from the filament. The emitted light is reflected by a sphereshaped glass cover.
Gas filled lamp
In the evacuated lamp, the filament evaporates and deposits inside of the glass cover after long use and makes black shade on the glass cover. To rectify this disadvantage, inert gases are filled in this lamp. Presence of inert gas causes heat loss. To compensate the heat loss, the filament is made as coiled wire. Increase in length of the filament leads to an increase in power.
a. Working principle of a f ilament or an incandescent lamp
As we know, when a room heater is switched on, it gives out red light with heat at the working temperature of 750oC. At this temperature, the radiations are produced by infra red rays. When an electric current is passed through a fine metallic wire, it raise the temperature of the wire. At low temperature, only heat is produced but at a higher temperature light radiation goes on increasing. The filament lamp consists of fine wire of high resistive material placed in an evacuated glass bulb. This type of lamp is operated at the temperature of 2500oC.
A tungsten filament is covered in an evacuated glass bulb. But to improve the life of the filament, some chemicals like argon or nitrogen or neon gases are filled.
b. Properties of metal for f ilament
1. It can be operated at a high temperature, since it has a high melting point.
2. It produces more heat because it has a high specific resistance.
3. Filament resistance may not change at the operating temperature because it has a low temperature co-efficient.
4. Because of low vapour pressure, it may not get vapour.
5. Because of high ductility, it may withstand mechanical vibrations.