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Thermodynamics

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Mercury Arc Lamp

As mercury volatilizes easily, a mercury arc can be obtained using mercury enclosed in a glass bulb. One of the forms of the mercury vapour lamp usually employed for spectroscopic purposes is given in fig.

It consists of two bulbs A and B connected by a quartz tube and the bulbs contain mercury. Two platinum electrodes are fitted in the bulbs as shown in fig. To strike the arc, a suitable potential is applied between the electrodes and the bulb A is gently titled so that a fine column of mercury makes contact with mercury in the bulb B. this provides the conducting path for the current and due to the heat produced, Hg vaporizes and this vapor produces the continuous conducting produces the continuous conducting path for the flow of current.

The current flows easily through the mercury vapour which glows. The light emitted from the arc is rich in ultra-violet radiation. Mercury vapour lamps are used as a source of ultra-violet light in medical therapy and spectroscopy.

In a recent form of mercury vapour lamp, the tube is surrounded by a water jacket and the vapour is kept under a high pressure. The luminosity of these lamps is very high (about 50 million candle power) and are used in search-lights etc.