Mercury vapour lamp
On the basis of pressure inside the discharge tube, the mercury vapour lamps are classified as High pressure mercury vapour lamp and Low pressure mercury vapour lamp. High pressure mercury vapour lamps are classified as:
1. MA type (Mercury vapour lamp with auxiliary electrode) These are operated at 220 ‐ 250 volt, AC supply and manufactured in 250 to 400 watts.
2. MAT type (Mercury vapour lamp with tungsten filament) These are manufactured between 300 to 500 watts and works at 200 to 250V (Both AC and DC)
3. MB type (Mercury vapour lamp with auxiliary electrode and bayonet cap) This type is operated at 200 – 250 volt, (AC and made in 80 watts and 125 watts)
a. Construction
It consists of hard glass tube enclosed in outer bulb of ordinary glass. The space between two bulb is completely evacuated to prevent heat loss by convection from the inner bulb. The outer bulb absorbs harmful ultraviolet rays. The inner bulb contains argon gas with a certain quantity of mercury. In addition to two electrodes, starting electrode having high resistance, connected in series is also provided. The main electrodes are made of tungsten wire in a helical shape. The lamp has a screwed cap and is connected to supply with a choke. A capacitor is connected across supply to improve power factor.
b. Working principle
The tube When the supply is switched on, full voltage is applied across main and starting electrodes. This voltage fills the gap between the electrodes and discharge through argon gas. As the lamp warms up, mercury is vaporized, which increases the vapour pressure. After 5 minutes, the lamp gives full light. It gives a greenish blue colour light. This lamp is always kept in vertical. Otherwise the inner glass tube may break due to excess heat.
c. Advantages
1. Mercury vapour lamps are more energy efficient than incandescent lamps.
2. It has high luminous efficacies of 35 to 65 lumens / watt.
3. It is durable. (in the range of 24,000 hours)
4. It has a high intensity.
5. It gives clear white light output which has made them ideal for outdoor use
d. Applications
1. Mercury vapour lamps are used in lighting applications.
2. It is used in streets and parking places.
3. It is used for landscape lighting.
4. It is used in factories.
5. It is used in gymnasiums.
