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Question
Draw a simple diagram of a Fortin's barometer and state how it is used to measure the atmospheric pressure.
Solution
Fortin barometer is used to measure atmospheric pressure.
Before reading the Fortin barometer, the free surface of the mercury in the reservoir is made to touch the tip of the ivory pointer by raising or lowering the level with the help of the screw S2. The vernier scale is then adjusted with the screw S1 till its lower edge touches the upper meniscus of mercury in the tube. While making this adjustment the eye is kept in level with the mercury meniscus. For this adjustment, the vernier is moved till the wall of the glass plate just ceases to be visible through the slits. The vernier can read to 1/ 20 mm or 1/ 500 inch. The reading of the barometer then gives the true atmospheric pressure. A thermometer is usually provided with the barometer which is also read and correction is made for the expansion of scale with a rise in temperature and also for the change in density of mercury with temperature. The scale graduations are correct only for the temperature at which the graduations are made.
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