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Sodium Vapour Lamp consists of a discharge tube created from a heat resistant glass, containing a small amount of metallic sodium, neon gas and two electrodes, Neon gas is added to commence the discharge and to create sufficient heat to vaporize the sodium. Simply because of law pressure inside the tube, a sufficiently extended tube needed to obtain far more light. To decrease the general dimension of the lamp, this tube is normally bent into U-shape.

The light created by this lamp is yellowish which is created at its optimum pressure of about .004mm of mercury. This pressure is obtained at a temperature of about 280 C and so it becomes needed to maintain this temperature. For this objective the U-tube is enclosed in a double walled flask to stop lose of heat. The double walled flask is interchangeable and can be fitted on to another U-tube. Although replacing the inner U-tube one must be really careful since if it is broken and sodium comes in get in touch with with moisture, it might outcome in fire.

All electric discharge lamps need a greater voltage at the time of beginning and low voltage for the duration of operation. Normally, sodium vapour lamps are operated by a high leakage reactance transformer. At starting a high voltage of about 450 volts is applied across the lamp which is enough to start the discharge. When the lamp is completely operative immediately after 10 - 15 minutes, the voltage across it falls to about 150 volts. Simply because of the high reactance of circuit, the power element is low and hence a p.f improvement capacitor is connected.

The efficiency of a low pressure sodium vapour lamp is quite high (about 40 - 50 lumens/watt) and it produces a light of particular wavelength possessing yellow color. Sodium lamps are primarily employed for street, high way and airfield lighting where color distinction is not so necessary.

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