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What is a High Pressure Mercury Vapor Lamp? â€“ Construction, Working and Applications
A gas discharge lamp that uses an electric arc through the vaporized mercury to produce light is known as mercury vapor lamp. The operation of the mercury vapor lamp is based on the excitation of atom of some gaseous medium. Also, the frequency and the wavelength of the radiation given out by different atoms of the gaseous medium depend upon the level of their excitation.
Construction of Mercury Vapor Lamp
The construction and circuit for a mercury vapor lamp is shown in the figure below.
It consists of an inner tube containing mercury vapor and it is made of hard glass.
It also has an outer glass cover which protects the inner tube from coming in direct contact with the atmospheric temperature variations and also absorbs the UV rays emitted by the lamp during operation.
The inner tube has a percentage of argon gas.
The lamp has two main electrodes viz. main electrode 1 & 2, which are made of tungsten wire and an auxiliary (or starting) electrode which is spaced very close to one of the main electrodes.
Working of Mercury Vapor Lamp
When the circuit of the lamp is completed, the supply voltage appears between the main electrode-1 and the auxiliary electrode. As the distance between the two electrodes is very small so the argon gas coming between these two electrodes is immediately ionized. As a result of the ionization of the gas between electrodes, a glow appears between the two electrodes.
A small current starts flowing through the starting resistor which is in series with the auxiliary electrode. This current results in building up of pressure due to heating up of mercury which is originally in the condensed form.
Ultimately, the gaseous medium between the main electrodes is ionized and the current starts flowing between the electrodes. It is no longer controlled by the high starting resistance but by the choke in the main circuit.
Also, there is a capacitor used in the lamp circuit which improves the power factor of the circuit which will otherwise be very low due to high inductance of the choke present in the circuit.
Advantages of Mercury Vapor Lamp
The chief advantages of the mercury vapor lamps are listed below.
These lamps produce a clear white light.
They provide the light with high intensity.
The mercury vapor lamps are energy efficient.
They are available in different shapes, sizes and ratings.
Mercury vapor lamps have relatively longer life span, about 24000 hours.
Disadvantages of Mercury Vapor Lamp
The disadvantages of the mercury vapor lamps are given as follows −
Mercury vapor lamps require 5 to 8 minutes warm up before fully glowing the lamp.
Mercury vapor lamps are very sensitive for the voltage.
Their maintenance of lumen is poor.
Their cooling time is about 5 to 7 minutes.
Applications of Mercury Vapor Lamp
Some areas of applications of the mercury vapor lamps are given as follows −
In households like garage, storeroom, etc.
In industrial areas
For street lightings
In security areas, etc.
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