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Why is the bore of the capillary tube of a mercury thermometer made narrow?


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2011-09-13 15:34:05
2011-09-13 15:34:05

Answer Vapour Pressure and Precision

dh25sThe vapour pressure of mercury (Hg) is low (0.0017 torr at 25 oC). The vapour pressure is dependent on temperature, thus what you want to measure. As the temperature increases, the vapour pressure increases and the mercury imbibes further up the capillary. If the capiallry radius were larger, the readings would be more difficult to see, i.e. the precision would decrease.

Vapor pressure has nothing to do with how a Mercury thermometer works. As the temperature rises, the mercury expands into a narrow bore, because the volume of the mercury increases. If the bore is very narrow then the extra volume has to go a longer way up the bore to accomodate that volume, so it is more sensitive to small changes of temperature.


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