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Lesson: Chapter 15 — Fluid Pressure and Its Applications

15.4. Gas and atmospheric pressure 5 of 8

Why the column holds at 76 cm

Why does the column stand at exactly 76 cm? The answer comes from a property we already know — pressures are equal at the same level.

The argument step by step

  1. The pressure at a point on the mercury surface outside the tube, in the vessel, equals atmospheric pressure.
  2. The pressure at a point inside the tube at that same level must therefore also be atmospheric pressure.
  3. The pressure at that point inside the tube arises from the mercury column above it — that is, P = hρg.
  4. So atmospheric pressure = hρg.

What if the tube is sunk deeper, or tilted?

Sinking the tube a little

The height of the mercury column does not change. All that happens is that the vacuum at the top becomes shorter.

Tilting the tube

The mercury appears to climb further along the tube, but measure the vertical height of the column and it is still 76 cm.

The reason is one and the same: pressure depends only on the vertical height of the liquid column. The length of the tube, its diameter, the volume of mercury in it and its tilt make no difference.

Pressure falls as you go up

At sea level atmospheric pressure is 76 cm Hg. Going up from sea level, the height of the column of air above you falls, so atmospheric pressure falls too.

Place Atmospheric pressure
Sea level 76 cm Hg
The summit of Everest about 25 cm Hg

Besides this, atmospheric pressure also varies with the weather — so even at one place a barometer reads a little differently from day to day.