The lamps in which the light is produced due to the how of electrons through the gas are known as the gas discharge lamps.
Mercury Vapour Lamp is a gas discharge lamp. It has high operating pressure with mercury vapours, so it is called as the high pressure mercury vapour lamp.
Figure 1: Mercury Vapour Lamp.
A Mercury Vapour Lamp is a type of gas-discharge lamp that produces light by passing an electric current through mercury vapor. It is widely used for lighting purposes because of its efficiency and bright light output. The lamp typically consists of an arc tube made of quartz or fused silica, which contains mercury and an inert gas (like argon) to facilitate ignition. The lamp requires a ballast to regulate the current.
Construction of Mercury Vapour Lamp
The Fig. 1 shows the construction of a mercury vapour lamp. It mainly consists of a discharge tube enclosed in an outer bulb of ordinary glass. The inner tube is made of hard glass or quartz. It contains two main electrodes at both sides of the tube, the main electrodes are made of tungsten. These are oxide coated to emit the electrons. There is an auxiliary electrode near the main electrode. This electrode is connected with the high resistance (50 kΩ) as shown in Fig. 1. The inner tube contains the argon and a certain amount of mercury. It is placed inside the outer tube. The space between both tubes is partially or completely evacuated to prevent heat loss from the inner tube by convection.
The two lead from the electrodes are connected to a bayonet or screw type cap. A choke is connected in series with the lamp to limit the current. The choke being inductive lowers the power factor, so a capacitor is connected in parallel to improve the power factor.
Working of Mercury Vapour Lamp
In the normal condition the gas is cool and mercury is in liquid form, so when the switch is on, the discharge between the main and auxiliary electrode takes place through the argan gas. The gas is heated up due to the discharge and becomes conductive. Now the mercury is also vaporised due to heating of argon gas. Moreover the mercury vapours behaves like a conductor and discharge takes place between the main electrodes. The current will not flow through the auxiliary electrode because of the high resistance (50 kΩ). Now the complete column will be heated up. The mercury vapours and gas increases the internal pressure of the tube. This lamp now works at a high pressure (2 to 3 atmospheric pressure) because of mercury vapours the light is whitish in colour.
Function of choke
The main function of the choke is to control the flow of current in the lamp. When the gas is cool its resistance is high and a low current flows through the gas. When it is heated up the resistance decreases and more current will flow. This more current may further increase but as choke is connected in series so a voltage is droped in the choke and the lamp actually works at low voltage and current is regulated to a safe value.
Power Rating of the of Mercury Vapour Lamp
The lamps are available in different wattage, 80 W, 125 W, 250 W and 400 W etc. The shape is either pearl or tumbler.
Uses of Mercury Vapour Lamp
The lamps are used for decoration purpose in big halls and for street lighting also.
Advantages of Mercury Vapour Lamp
- High Efficiency: Mercury vapor lamps are more efficient than incandescent lamps and produce more lumens per watt.
- Bright Light Output: They emit a bright white or bluish light that provides excellent visibility.
- Long Lifespan: These lamps have a longer operational life compared to many other types of lamps.
- Cost-Effective: They have lower operational costs due to their efficiency and longevity.
- High Intensity: Suitable for large areas as they provide high-intensity illumination.
Disadvantages of Mercury Vapour Lamp
- Warm-Up Time: Mercury vapor lamps take a few minutes to reach full brightness after being turned on.
- Poor Color Rendering: They have limited ability to accurately display the colors of objects under their light.
- Environmental Concerns: Mercury is toxic, and improper disposal can harm the environment.
- UV Radiation: They emit ultraviolet (UV) radiation, which can be harmful without proper shielding.
- Replacement Issues: When the lamp degrades, it can shift in color or reduce its light output significantly before failure.
Applications of Mercury Vapour Lamp
- Street Lighting: Commonly used for illuminating roads and highways.
- Industrial Lighting: Used in factories and warehouses due to their high-intensity light.
- Sports Venues: Suitable for stadiums and arenas for nighttime events.
- Parking Lots: Ideal for large outdoor areas.
- Landscape Lighting: Used for illuminating gardens and parks.
- Commercial Spaces: Found in shopping malls and commercial buildings.
- Aquariums and Terrariums: Used for UV light generation in certain settings.
General instructions of Mercury Vapour Lamp
- The working temperature of the lamp is 600°C.
- It can be used only in vertical position with cap up.
- It can be used only on A.C.
- It requires sometimes to come to full light.
- It must be used with a choke of suitable wattage.
- It has whitest and smooth light.
- The correct identification of colour is not possible in this light.