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Working Of Gas Thermometer




 A small amount of  gas is enclosed in bulb which is in communication via the capillary tube with one limb of  the mercury manometer M. The other limb of  the mercury manometer is to open to the atmosphere and can be moved vertically to adjust the mercury levels so that the mercury just touches lip L of the capillary. The pressure in the bulb is used as a thermometric property and is given by 

                                    p = p0 + pM Zg

where p0 is the atmospheric pressure, pM is the density of  Mercury.

When the bulb is brought in contact with the system whose temperature is to be measured., bulb in course of time, comes in thermal equilibrium with the system. The gas in the bulb expands, on being heated, pushing the mercury down ward. The flexible limb of  the manometer is then adjusted so that the mercury again touches the lip L. The difference in mercury Level Z is recorded and the pressure p of the gas in the bulb is estimated. Since the Volume of  the trapped gas is constant, from the ideal gas equation, 

                                       T* = (V/R ) P*

i.e., The temperature increase is proportional to the pressure increase.

In a constant pressure gas thermometer, the mercury levels have to be adjusted to keep Z constant, and the volume of gas V, which would vary with the temperature of the system, becomes the thermometric property.

                                          T* = (P/R) V* 

i.e., The temperature increase is proportional to the observed volume increase. The constant volume gas thermometer is, however, mostly in use, since it is simpler in construction and easier to operate. 

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