A Constant Volume Gas Thermometer?

Concerning the mechanism of a constant volume gas thermometer, I was wondering how the adjustment of the second tube upon the open side increases the pressure; specifically, I possess knowledge of how the gas placed in the sample places pressure upon the mercury. However, provided my small quantity experience within physics, I do not know how manually raising the tube on the non-sample, open side increases pressure; is there increased gravitational pressure on the mercury or does the mercury push downwards to compensate? If it is the latter, what is the explanation behind this? Thank you for your assistance.

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  • 1 month ago

    When you lift the tube on the open side the mercury starts to lift slightly.  This causes an increased pressure DIFFERENCE so the mercury flows from this side back towards the gas side.  This decreases the volume of the gas.  You continue to lift the open arm until sufficient mercury has flowed back so that the level in the fixed branch is brought back to your reference mark of zero.  This guarantees that the volume of gas remains at its standard value.

    The final pressure differential is caused by the difference in height of the mercury column on each side.  The higher side causes more pressure than the lower side.  As the total pressure must be equal on each side when in equilibrium then the pressure of the gas is equal to the difference in the pressure of the mercury between the two sides.

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