The

open-tube manometer is a device for measuring

pressure. It is a

U-shaped tube,

partially filled with

Mercury. We can find the

atmospheric pressure by finding the difference in the heights of the liquid in each '

branch' of the tube. The pressure is related to the difference by the formula:

*P = P*_{0} + ρgh

where P_{0} is atmospheric pressure and ρ is the density of Mercury at whatever temperature you are performing the measurement at. *ρgh* is known as the guage pressure, obviously the difference between atmospheric pressure and P. However, often only the difference in heights is given, in a unit known as millimetres of Mercury (mm-Hg). This unit of measure is also known as the torr, after 17th century chemist, Evangelista Torricelli. The SI unit of measure of pressure is the Pascal or Newtons per metre.