Construction:- It consists of an inner bulb generally of silicon, to withstand high temperatures. The bulb contains a small quantity of mercury and argon. It is protected by outer glass, this may be cylindrical or elliptical. The space between the two bulbs is filled with nitrogen at a pressure of half atmosphere. The discharge tube has three electrodes, namely two main electrodes A and B and one starting electrode. The starting electrodes are connected through a resistance of about 10-30 k ohm to the main electrode, located at the far end. The electrodes are of tungsten wire helices filled with electron emissive materials, usually barium and strontium carbonates mixed with thorium.
The construction & connection diagram is as shown in figure. As per this construction there are following components.
Choke: The choke is acting as the ballast. At the time of supply voltage variation of current flowing through the inner tube is maintained constant to keep uniform light intensity. Sometimes choke can be designed for to get the higher voltages & to apply the inner tube of mercury vapour lamp. Starting resistance/limiting resistance: Whenever current flows through the starting resistance there is a I2R loss which is converted into heat. If the temperature of this heat goes near about 6000C then there will be heating effect & inert gases ionization will be start. Auxiliary electrode & Main electrode: It is made by high resistive element. The ionization is taking place through the inert gases whenever current flows from auxiliary electrode to main electrode. Inner Tube: The various inert gases e.g. Argon, Nitrogen etc with mercury powder are filled in the inner tube at low pressure or high pressure. Outer Tube: The function of outer tube is to make the vacuum surrounding the inner tube to avoid thermal dissipation or to maintain 6000C surrounding the inner tube. Power factor improvement Capacitor: The function of power factor improvement capacitor is to improve the power factor 0.5 to 0.95