Final answer:
When the level of liquid in the right-hand side of a manometer decreases, it indicates higher pressure on that side due to the principle of pressure equilibrium. Negative pressure applied to the left-hand side can also cause this effect, as well as changes in the conditions such as temperature affecting the gas volume. Capillary action can affect readings, but is minimized in uniform diameter tubes.
Step-by-step explanation:
The behavior of the fluid in a manometer is based on the principle of pressure equilibrium. When both sides of the manometer are open to the atmosphere, the pressure on both sides is equal. Due to this equilibrium, the fluid reaches equal levels on either side, regardless of the tube diameters.
During an experiment, if the level of liquid in the right-hand side of the manometer went down, it suggests that the pressure in that side was higher than the pressure on the left-hand side, causing the liquid to move towards the side of lower pressure.
If negative pressure is applied to one side of the manometer, for instance the left side, as stated 'Applying negative pressure to the left side lowers Up, causing water to move to the left side of the tube', the fluid moves towards the left side, resulting in the right-hand side's liquid level going down.
Capillary action can also influence the manometer reading in smaller diameter tubes by altering the height of the fluid column; however, with uniform diameter tubes, this effect is minimized.