What is barometer?

What is the height of barometer?

• as we know height of mercury in barometer is related to weight of air on area of reservoir but if increase the area of reservoir will it change the reading of barometer.Is there any standard area for reservoir if you do'nt understand question just tell me.

• Answer:

  Well ... the average pressure at sea level is roughly a kilo per square centimeter. If you extend your thumb that is roughly one square centimeter, do you feel one kilo pressure on it? Of course, not! The point is that the pressure on your thumb and entire body comes from all directions since air is a fluid. If you could remove the air pressing from under, you could then measure the atmospheric pressure and that can be simply done by using a heavier fluid like, water, in a contained tube. If you do so and close the tube at one end, the tube will have to be 10 meters high to create vacuum (thus equal pressure) at the closed end. The diameter of the tube is irrelevant! It is the pressure as weight per square surface. If you increase the surface, you will increase the weight, of course. But the column will remain ten meters high! ... incidentally, this is why a suction pump cannot lift water over ten meters! Having a barometer ten meters high is not practical so we use a fluid that is much heavier, mercury. Using that, the tube can be reduced to 760 millimeters (29.9 inches) and this is why the USA, still not adopting the metric system, quotes often the atmospheric pressure as inches of mercury. Elsewhere, the pressure was measured in millibars, which is a thousandth of a bar, a.k.a. one atmosphere. The average pressure on earth at sea level is defined by the International Standard Atmosphere (ISA) as 1,013.25 millibars but today, we call it, hecto Pascal or hPa. So, if you read the Standard Atmosphere, it will say that the average pressure is 1,013.25 hPa, the average temperature is 15 C and the average adiabatic lapse rate is 0.65 C per 100 meters. Incidentally, did you know that an aircraft altimeter (what measures the altitude) is actually a barometer? Of course, not a mercury one but a aneroid one with a pressure chamber. This is why the first thing a pilot do, prior to take-off, is to adjust the altimeter for the local atmospheric pressure; what is called the QNH. Then his altimeter matches e.g. the elevations over sea level of surrounding obstacles.