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Thermodynamics

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Fluroscent Lamps

A fluorescent lamp consists of a long glass tube coated on the inside with a fluorescent material (calcium and magnesium tungstates or zinc and cadmium silicates depending upon the colour of light desired). The electrodes E₁ and E₂ are two coils of wire fitted at the two ends. The current flows through the mercury vapour contained in the tube and ultra-violet radiations are emitted from the walls of the tube. The current and voltage can be suitably regulated so as to make the emitted light rich in ultra-violet radiation.

The thermal switch is used to start the tube. S is a bimetallic strip. Initially the current passes through S into E₁ and E₂. This high current raises E₁ and E₂ to incandescence. The nichrome resistor R also gets heated. The temperature within the bulb G rises and the bimetallic strip S bends. Thus the contact is instantaneously broken. This induces a high potential of the order of 1000 volts across E₁ and E₂ and ionization of argon takes place. The fluorescent tube gets warmed up and the mercury volatilizes and the discharge passes continuously between the electrodes.