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.
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.
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.