# Understanding Millimeter of Mercury

A Millimeter of Mercury(mmHg) is by definition the pressure exerted on a column of mercury 1 mm in height.

I found this definition confusing because it doesn't say how the diameter of the container will affect the pressure.

## 1 Answer

Pressure is defined as force exerted per unit area. Therefore, the diameter of the mercury column is taken into account when we calculate the area of the column. A mercury column with a large diameter will exert a greater force than a column of the same height, but with a smaller diameter - but the force divided by the area will be the same.

• The different in height(u shape manometer) doesn't depend on the diameter of the tube. But with fix amounts of force, increasing diameter will reduce the difference in high – most venerable sir Oct 5 '14 at 20:46
• So you mean the pressure is the constant? – most venerable sir Oct 5 '14 at 20:49
• My textbook just told me diamt has nothing to do with height difference, but how can the height not decrease with diameter decreasing if the volume of mercury stays the same? – most venerable sir Oct 5 '14 at 20:53
• Well, diameter does have something to do with it, it is part of the equation for the area. But because diameter is included in the equation for pressure, we get the same pressure no matter what the diameter is. A larger diameter will have more force on it (bigger area), but the force per unit area will be the same even if we use a smaller diameter. – ron Oct 5 '14 at 21:06