# Size of mercury barometer and effect on its reading

I was thinking that since atmospheric pressure is 760mmHg what would happen if I shrink a mercury barometer until it's shorter that 760mm in height. What would happen? Would the barometer retain it's ratio of mercury height? Will the mercury fully fill the barometer? Is 760mmHg a constant and not affected by the size of the barometer (or diameter)? Thanks in advance.

• You should probably first make more of an effort to try to understand how a mercury thermometer works.
– user93237
Commented May 7, 2017 at 5:51
• I think that if you if u decrease the height of the barometer to less than 75 mm then the mercury will raise to the top such that the top of the tube wil exert an pressure such as to compensate for the low pressure due to capillary rise.
– Pink
Commented May 7, 2017 at 6:05
• Samuel, I think I do know how a mercury thermometer works. (Temperature rises mercury expands mercury level rises (correct me if I'm wrong)) but the difference is that you can find mercury thermometers in different shapes and sizes, and they all rise in different amounts depending on how much mercury is used and the diameter of the bore. However for a mercury barometer the value for pressure is it's height (exact value of 760mm) so I'm not sure if you can shrink it and still have it function. Commented May 8, 2017 at 3:49
• Navinstudent, ohhh so this means that the barometer will just fill up to the top and instead of negative pressure now the tube is pressurized ? (I assume in normal cases the top of the mercury barometer should be in negative pressure as it is vacuum there ) Commented May 8, 2017 at 3:51

A mercury barometer is designed to measure the difference in pressure between the surface of the mercury outside the tube and the surface of the mercury inside the tube.
The space inside the tube is filled with mercury vapour and hopefully nothing else.
The column of mercury is kept in position because the pressure exerted on the surface of mercury outside the tube is equal to the pressure inside the mercury column at the same horizontal level as the mercury surface outside the tube.

If the tube is less than $760 \, \rm mm$ then there will be insufficient height of mercury to equate the pressures even if the mercury column occupies the whole of the tube.
In such a case the wall of the glass tube exerts downward forces on the mercury inside the tube such that the pressure exerted on the surface of mercury outside the tube is equal to the pressure (due to the column of mercury and the tube) inside the mercury column at the same horizontal level as the mercury surface outside the tube.

• Farcher, this means that the barometer will just fill to the top right? I assume that the barometer will not burst as now it just looks like an inverted aquarium. boredpanda.com/inverted-aquarium-pond-luxusburger Commented May 8, 2017 at 3:54
• Yes, the mercury will fill the barometer tube which has relatively thick glass walls. Commented May 8, 2017 at 5:58

A vacuum must exist above the meniscus of the column so that the mercury can rise above 760mm (or 30 in.) which is what a mercury barometer reads at sea level. In other words, the mercury column needs headroom to rise with increasing ambient atmospheric pressure.

A barometer capillary tube with a ball pediment cistern must have an overall length of 33 inches to allow for the column to rise to 31 inches (which is rare here in the Midwest, but may read higher if your location is below sea level). Search "barometer ball pediment tube" for clarification.