The Barometer

A barometer might take either of the forms shown in figure below; in both situations, the space above the liquid in the closed arm is emptied. When the pressure in the open-ended arm is p and the difference of height of the two liquid columns is z, as the pressure in the closed end is zero,

                                                                    p = ρ gz     

Therefore, the barometer is an instrument that measures absolute pressures, not pressure differences.

                                                                   Figure: Barometer