Atmospheric pressure is the weight of the air pushing down on everything. A traditional mercury barometer measures this push using a closed glass tube. The tube sits upside down in a pool of liquid mercury.
The heavy air pushes down on the pool. This forces the liquid mercury up into the empty tube. The height of that liquid column directly shows the air pressure.
When pressure rises, the mercury climbs higher. When pressure drops, the liquid falls. Modern aneroid barometers do not use any dangerous liquid.
They use a sealed metal chamber instead. This chamber expands or shrinks as the outside air pressure changes. Mechanical springs measure these tiny movements.
Barometers are absolutely essential in chemistry labs. Atmospheric pressure significantly changes the volume of any gas. It also alters the boiling points of liquids.
It can even speed up or slow down chemical reaction rates. Recording the exact air pressure during an experiment is crucial. It ensures other scientists can reproduce the results accurately. It lets researchers compare their findings under standard conditions.