This lamp is also known as an electric discharge lamp in which light is produced by gaseous conduction
Construction :
It consists of two glass tubes and the arc is confined to a small glass tube which is the inner one.
The larger glass tube surrounds the inner glass tube and the space between these two tubes is completely evacuated so that the heat losses are eliminated.
The inner tube contains an organ, and in addition to a small quantity of mercury.
The inner tube also contains three electrodes out of that two are main (A and B) and an auxiliary electrode (C).
Main electrodes are made up of tungsten coils coated with barium oxide.
The auxiliary electrode is connected to electrode 'B' through resistance R but at the opposite end as shown in Fig.
A choke limits the current and connected in series with the lamp.
A capacitor is connected in parallel and used to improve the power factor.
Inner side of outer tube has a coating of a fluorescent material since the spectra of mercury is deficient of red colour. This corrects the colour of emitted light.
Working:
When supply is switched on an initial discharge is established in the organ gas between electrode A and electrode C and then in the region between electrode A and electrode B.
The heat is produced due to the discharge through gas which causes the warming up of the inner lamp.
Thus the mercury gets vaporised and increasing its pressure and thus the light output.
It takes about 4-5 minutes for the mercury are to build up to produce full light output.
After 45 minutes mercury vapours emit greenish-blue light.
Precaution must be taken that, once the mercury lamp is switched off, it can be restart after 8 to 10 minutes (since it requires cool down to reduce repair pressure before restart).
Mercury vapour lamps are available in various sizes upto 2 kW and their efficiency is about 50 lumens per watt frequency 50 Hz
Specifications: M.A. type, voltage 250 to 400 V, wattage - 250 to 400 watt, supply A.C.
Another precaution is that these lamps must be operated vertically if they are used horizontally convection causes the discharge to touch the glass, which eventually will fail.
Applications:
centres, industrial installations, etc.
These lamps are widely used in street lighting, railway yards, stadium floodlighting, shopping