# 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. – Samuel Weir May 7 '17 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 May 7 '17 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. – See Jian Shin May 8 '17 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 ) – See Jian Shin May 8 '17 at 3:51

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.