INVENTOR OF MERCURY VAPOUR (MV) LAMP
John
Thomas Way (London) developed a mercury vapor lamp in
the year 1860. He tested them on the Hammersmith Bridge in London.
The
modern high-pressure mercury vapor lamp is developed in the year 1936 by
Philips (Nederland) No individual names are available.
The
mercury vapor lamp is a High Intensity Discharge (HID)
Lamp
and it is also called as hot cathode gas-discharge lamp.
HPMV
lamp is similar in construction to the sodium vapour lamp and may have low
pressure in the bulb.
A
high pressure (1 to 2 atm) mercury vapour lamp has amount of mercury introduced
in the lamp.
A
recent development in HPMV lamps in an ultra-pressure lamp operating at about 40
atmosphere and its efficacy is more than the sodium vapour lamps.
1.
It consists of a discharge envelope enclosed in an outer bulb
of ordinary glass.
2.
The discharge envelope may be of hard glass or quartz.
3.
The space between the bulbs is partially or completely
evacuated to prevent heat loss by
convection from the inner
bulb.
4.
The inner bulb consists of argon and a certain quantity of
mercury and outer tube is coated with
fluorescent material.
[Argon is introduced for initial discharge
of the mercury]
5.
The outer bulb absorbs harmful ultra violet rays.
6.
This lamp consists of two main electrodes and one auxiliary
electrode.
7.
The main electrodes are made up of tungsten wire in the
shape of helixes and it containing a
coating of elements of
thorium material (electron emitting).
8.
An auxiliary or starting electrode is connected through a high
resistance.
9.
A choke coil having different tapping is connected in series
with the lamp to give high starting voltage
for discharge and
for controlling the current and voltage
across the lamp after
discharge.
10.
A capacitor is connected across the supply to improve the
power factor.
OPERATION
1.
When the supply is switched on, full main voltage is applied
between the auxiliary electrode and
neighboring main
electrode.
2.
When the tube is filled with mercury vapor, low resistance
path is created and arc shifts between main
electrode and
discharge tube.
3.
Current does not flow through the main electrodes due to
high resistance of the gas.
4.
However the current starts to flow between the main
electrodes and the auxiliary electrode
through the argon gas.
5.
This breaks down the gap and a discharge through the argon
takes place.
6.
This enables the main discharge to commence.
7.
The discharge from the tube strikes the fluorescent coating
of outer bulb from which secondary emission
starts and gives
crisp white color light.
8.
The vapour pressure rises until all the mercury is vapourized
when the light given out is with a bluish
tinge (tinge means
a slight but appreciable amount).
9.
This lamp is like a fluorescent lamp, requires a ballast
designed for its specific use.
10.
Special ballasts are required for dimming.
ADVANTAGES
1.
Color rendering is better than that of high pressure sodium
street lights.
2.
Long life (16,000 to 24,000 hours).
3.
It produces similar to day light which may help in increasing
the production of a factory.
DISADVANTAGES
1. HPMV Lamps are the most inefficient source of light, with an
efficacy in the 25 to 55 lumens per watt
range.
2. HPMV lamps may be greatly affected by lamp lumen
depreciation and should be replaced after
24,000 hours.
3.
Four to five minute cooling and restart time is needed.
4.
Like many lamps it contains traces of mercury which must
be disposed of properly.
5.
It takes 6A approximately when switched on and after six
minutes it falls to 3A.
APPLICATIONS
1.
These lamps are suitable for large areas like parks, street
lighting, high ceiling buildings, and gyms.
2.
MV lamps have found greatest use in industrial applications
and outdoor lighting.
3.
Lamp sizes range from 40 to 1,000 watts.
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