Mercury Vapour Lamp and Mercury lodide Lamps

Mercury Vapour Lamp :
Q. Explain with neat sketch construction and working of high pressure mercury vapour lamp.

Principle : “When mercury discharge under low pressure gives mainly ultraviolet radiations. If pressure is increased to one or two atmospheres, its proportion of radiations in visible spectrum is increased and we get light having bluish tinge".

Construction and Working :
  • It consists of two tubes. Inner tube contains neon or argon gas under low pressure . two main electrodes in the form of oxide coated coils and starting or auxiliary electrode near the cap end side of main electrode.
  • Inner tube is made of hard glass and in an evacuated outer tube, which maintains high operating temperature of the inner arc tube.
  • When supply is given , electric field is setup between the starting electrode and the adjacent main electrode. This causes discharge first to take place between them through limiting carbon resistance.
  • This discharge through argon gas vaporize all mercury and for the lamp to reach full brilliance requires 4 to 8 minutes depending upon the design.
  • Once arc tube is filled with mercury vapour a low resistance path is created for current to flow between the main electrodes. The are then shifts to main electrodes. Path of starting electrode automatically becomes inactive.
  • Electrodes are kept in electron emitting condition by the bombardment of heavy mercury ions.
  • Should the lamp go out while in services , it will not restart till mercury vapour pressure has fallen low enough to allow for restarting of discharge between main and auxiliary electrodes
- Mercury discharge lamps are made in five sizes viz. 80, 125, 250, 400 and 1000 W.
- In case of mercury vapour lamps, magnitude of the striking, voltage is very much affected by ambient temperature.
- Efficiency of these lamps is of 30 to 40 lumen/watt and life is about 5000 hr.

Application : These are used for high ceiling manufacturing bays street lighting , stage lighting , sport grounds , garages and petrol station lighting , railway marshalling yards and park lighting

Mercury lodide Lamps :
  • Construction is same as that of the HPMV lamp.
  • In HPMV lamp only mercury is added where as in mercury iodide lamp mercury and also number of iodides are added for filling the gap in light spectrums to improve the colour characteristics of light so produced.
  • As compared to HPMV lamp its efficiency is higher (70 to 90 lumens/watt)
  • In HPMV lamp for ignition only choke is used where as in mercury iodide lamp Choke as well as a separate ignition device is used.
Advantages :
  1. Good light quality
  2. Higher efficiency (70 to 90 lumens/watt )
  3. Better performance
  4. Ignition is effective
Applications :
  1. In the fields of flood-lighting
  2. Industrial lighting
  3. Public lighting

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