What is a barometer? How is a simple barometer constructed?

A barometer is an instrument which is used to measure the atmospheric pressure.

A simple barometer uses mercury as the barometric liquid.

Construction — It uses a hard glass tube of about 1 m closed at one end. The tube is completely filled with pure mercury such that no air bubble remains inside the tube. The open end of tube is closed with thumb and the tube is then made upside down several times so as to force out any air bubble which might have entered in it.

The completely filled tube with it's open end closed by thumb is then inverted into a trough of mercury in such a way that the open end of tube is well immersed in mercury in a trough and the tube stands vertical as shown in figure. Now the thumb is removed. Care is taken that no air enters in the tube.

It is seen that the level of mercury in the tube falls till it's height above the level of mercury in the trough becomes h (nearly 76 cm) as shown in the diagram below.