The following points highlight the three main types of filament lamps used for domestic and commercial purposes. The types are: 1. Carbon Lamp 2. Metallic Filament Lamp 3. Coiled-Coil Lamp.
Type # 1. Carbon Lamp:
The filament used in this lamp is made of carbon. Carbon is of the nature of coal. Air is pumped out of the bulb. Owing to How of current when the filament gets heated, its temperature rises to about 1,800°c. At this temperature the filament becomes whitish red and emits light.
Carbon, when heated, burns out. But the carbon filament does not burn out as the bulb in which it is placed is vacuum and contains no air. What is commonly known as ‘burning’ is actually a chemical reaction between burning substance and the oxygen of air. Inside space of a bulb is practically vacuum. Hence, carbon or any other metal filament cannot burn out.
Nowadays much improved types of filament lamps have been developed. As a result carbon lamps are no longer in general use.
Type # 2. Metallic Filament Lamp:
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When a wire of tungsten or similar metal is placed inside a glass bulb and all air is drawn out from it, a vacuum metal filament lamp is obtained. When current is passed through such a lamp, filament is heated and becomes white hot. At this stage temperature of the filament is about 2,000°c.
Hence, the total illumination available from such a lamp is more than that obtained from a carbon filament lamp. All lamps with zigzag filaments are usually of vacuum type. Metal filament lamps are available for these supply voltages—35-55; 60-75; 100-130; 200-260 volts. There is no difference in lamps used in a.c. circuits and d.c. circuits. Lamps available in the market are usually rated from 15 watts up to 1,500 watts.
Gas-filled lamp and vacuum lamp:
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In a vacuum lamp air is drawn out from the inside space of the glass bulb and the space is left vacuum. But in a gas-filled lamp the vacuum space is again filled with some inert gas such as nitrogen, argon, crypton etc. which are inert and do not help combustion. This gas helps to raise the temperature of the filament further.
In a gas-filled lamp filament is heated to white hot and it becomes so bright that one cannot look at it with naked eye. The filament is usually fitted horizontally in-a crescent form. For the same intensity of illumination the gas-filled tungsten lamp consumes less power than vacuum metal filament lamp.
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The gas inside the bulb ‘is not only inert, but it makes easier the loss of heat by convection currents. As a result the efficiency of the lamp increases, i.e. the capacity of the lamp to produce more illumination for a given amount of power consumption increases.
Type # 3. Coiled-Coil Lamp:
In this lamp the filament is at first coiled to form a long narrow spring and then this spring length is coiled again. For this reason the lamp is known as ‘coiled-coil lamp’. The filament wire of this lamp remains in a very compact form resulting in more heat and light. The manufacturers of coiled-coil lamps claim that, in comparison to single coil lamp, about ten to twenty per cent more illumination is obtained from a coiled-coil lamp for the same power consumption.
