What instrument is used to measure air pressure?
A barometer using mercury is common. The pressione atmosferica presses down on a reservoir of mercury and the mercury is forced up the tube a certain distance depending on the pressure. This is often recorded as #"mm"# #Hg# or (rarely) #"inches"# #"Hg"#.
Our lab uses meters with a digital display to measure the pressure and I suspect that they use a method similar to an aneroid barometer which uses wafers that expand and contract with pressure changes to move an instrument. More can be read here: http://www.windows2universe.org/earth/Atmosphere/measuring_press.html