Final answer:
A manometer measures the pressure of gases by the difference in height of a liquid column in a U-shaped tube. Option A is correct.
Step-by-step explanation:
A manometer is a device used to measure the pressure of gases. It consists of a U-shaped tube containing a nonvolatile liquid, typically mercury due to its high density.
When connected to a gas sample, the pressure exerted by the gas on one column of mercury creates a difference in height (denoted as h) in the two columns of the U-tube. This difference in height is directly proportional to the pressure of the gas.
There are two types of manometers: a closed-end manometer, where one arm is closed and the height difference measures the absolute pressure of the gas, and an open-end manometer, where one arm is open to the atmosphere and the height difference measures the gauge pressure relative to atmospheric pressure.