The atmospheric pressure is measured most accurately with the help of a mercurial barometer. For checking the accuracy of other types of barometers, the mercurial barometer is considered as standard.
In 1643, Evangelista Torricelli, a student of Galileo, made an experiment that formed the basis for the development of mercurial barometer, an instrument which measures atmospheric pressure accurately.
He took 3 feet-long glass tube filled with mercury. He kept the top of the tube closed and inverted it into an open vessel filled with mercury. He opened the top of the tube only when it was beneath the level of mercury in the vessel.
To his surprise he saw that mercury in the tube fell until it was at a height of about 30 inches above the level of mercury in the pan, leaving a vacuum at the closed end of the tube.
What happened was that the pressure exerted by atmosphere on the open pan filled with mercury was equal to that of mercury in the tube. The rise and fall of the atmospheric pressure is indicated by a corresponding rise and fall of mercury column in the tube.
The standard sea-level pressure is given as 1013.25 millibars at a temperature of 15″C and at latitude of 45°. This pressure is equivalent to 29.92 inches or 760 millimeters.
The millibar is a force equal to 1,000 dynes per cm2, and a dyne is a unit of force approximately equal to the weight of a milligram.