A manometer is an instrument that measures the pressure of a trapped gas or liquid. It also measures the pressure difference between two different points. It does this by showing the height of a liquid column inside a clear calibrated tube.
In more detail
Manometers rely on a simple rule called hydrostatic pressure balance. The liquid inside the tube moves until the pressure it exerts perfectly matches the outside push. The most common design is the open-tube manometer.
It features a U-shaped glass tube partially filled with liquid. One end connects to a sealed flask containing the gas you want to measure. The other end stays open to the normal room air.
The room air pushes down on one side of the liquid. The trapped gas pushes down on the other side. The liquid level drops on the side with the stronger push.
Mercury is the traditional measuring liquid for these devices. It has a very high density of 13.6 grams per cubic centimeter. This extreme density allows chemists to build short and compact instruments.
Mercury also barely evaporates at room temperature. This keeps the measurements highly accurate and stable over time. For measuring tiny pressure changes, chemists might use water or light oils instead.
These lighter liquids move much further than mercury under the same small pressure. This creates a more sensitive reading.