Mercury vapour light: It is a gaseous lamp that consists of two bulbs, one within the other. The inner bulb is called the arc tube, which is made of quartz and the outer bulb serves to protect the are tube. The inner bulb contains a small amount of argon and mercury, a starting electrode S is placed very close to the two main electrodes E1 and E2 and E1 is an auxiliary electrode connected to the other electrode E2 through a high resistance R of about 500 ohms. Each of the main electrodes E1 and E2 have a tungsten wire spiral containing a rod of barium compound that provides an ample supply of electrons when heated. When the lamp is turned on, the full mains voltage is applied to electrodes E1 and S so that the discharge goes between the two, which initiates the ionization of the gas. When the lamp is heated, the mercury slowly evaporates and the pressure increases. The mercury discharge between E1 and E2 fills the entire tube. Since the resistance of the initial circuit is much higher than that of the parallel circuit, argon discharge is ineffective.
The discharge tube is made of quartz instead of glass so it can withstand high pressures. The tube is about 5 cm long and is housed inside a pearl glass bulb like a simple 100-watt lamp. Such lamps are commonly used in laboratories.
The lamp is worked on AC mains with the help of a choke which serves as a stabilizer. A condenser is also placed across the mains to improve the efficiency of the lamp. If the A lamp is switched off it cannot be started again at once because mercury must condense before the starting electrodes can function. A mercury-vapor lamp lasts longer than electric lights of similar wattages. But, mercury vapor lamps require five to seven minutes for it to reach its full brightness.
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