Operation Of 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. 
I know how the gas placed in the sample places pressure upon the mercury. However, 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.
 A: 
Suppose that from the diagram the temperature of the gas is increased. This increases both the volume $V$ and pressure $p$ of the gas. The manometer level on the left dips below the reference mark, while that on the right rises higher. The difference $h$ in liquid levels in the manometer increases from $h$ to $h_1 > h$ because the gas pressure has increased.
In order to make a constant volume measurement the liquid level on the left must be raised back to the reference mark. To do this the tube on the right is raised up. This brings more liquid on the right above the level on the left. The extra liquid comes from the flexible tube, the bottom of which moves up by half as much as the tube on the right. (Imagine cutting out 1cm of flexible tubing on the left and inserting it on the right instead.)
Suppose we raise the tube on the right a little. This increases the difference in liquid levels even further. The pressure exerted on the gas by the manometer liquid on the left increases, compressing the gas again, moving the liquid level up on the left. The pressure in the gas has increased so the difference in manometer liquid levels increases further to $h_2 > h_1$. 
We keep adjusting the height of the tube on the right until the liquid on the left is back at the reference mark. Moving the tube on the right changes both the level on the left and also the difference in the liquid levels. This reflects the fact that we are changing the volume $V$ and the pressure $p$ of the gas at the same time, in such a way that $pV=\text{constant}$. 
When the liquid on the left is back at the reference mark the volume of the gas is back to its original value and its pressure has been increased to a final value where the difference in liquid levels is $h_3 > h_2$. We can then read off the corresponding temperature on a calibration chart or table.
