Atmospheric Pressure - Page 9
Mercury Barometer

The mercury barometer works on the principle of balancing the weight of a column of air by the weight of a column of mercury.

The weight of the column of air pressing down on the mercury in the cistern is equal to the weight of the mercury which has been pushed up the measuring tube by that air pressure on the mercuty. Thus, the length of the mercury in the tube becomes a method of measuring the pressure of the atmosphere, i.e., 29.92 inches of mercury or 760 mm of mercury.

Aneroid Barometer

The aneroid barometer was developed because of the difficulty in using a mercury barometer. Aneroid means "without fluid." The aneroid barometer consists of four basic parts.
  • The first part is a closed chamber (the aneroid chamber-#1) of thin, pliable metal, partly evacuated. Thus, it is not "truely" aneroid. This chamber expands as the atmospheric pressure pressing on the chamber decreases in magnitude. The chamber compresses as the atmospheric pressure increases in magnitude. The second part, a spring, #2, either within the chamber or external to the chamber, as in the image to the right, is used to keep the chamber from collapsing. One side of the chamber is usually stationary and the other side is allowed to move under changing air pressure. The third part transfers movement of the chamber and is a linking mechanism, #3 - #9, to the fourth part, a pointer on the face of the aneroid barometer or to a pen to record on a moving graph the atmospheric pressure.

The aneroid barometer operates on the same principle as the mercury barometer - balancing the weight of the atmosphere, - but differs in that instead of balancing against a column of mercury, it balances against the combined forces of the spring and the residual air within the chamber.

Capacitive Resistance Barometers These barometers work in a manner similar to an aneroid barometer. Two small plates in an electric circuit either move closer together (when atmospheric pressure increases) or farther apart (when atmospheric pressure decreases). This causes a change in the resistance to electric current in the circuit. When the plates are close together, resistance is low and the current flow is greater. When the plates are farther apart, resistance is high and the current flow is less. The current flow in the circuit can then become a measure of the atmospheric pressure that caused a change in resistance in the circuit. We will use this type of barometer to make some pressure measurements.

Problem 8.Using the barometer provided, measure and record the atmospheric pressure (with the barometer sitting on the floor) at the first and twelfth floors.

Make your measurement in millibars.
Record your measurements on the answer sheet

Now, determine the weight (in pounds) of the column of air which extends from the first floor to the twelfth floor of this building. To do this, convert the difference in pressure in millibars to a difference in pressure in lb/in2. Then, determine what the force (in pounds) would be if the area were 1 ft2 rather than 1 in2.

Show all your work on the answer sheet.

Problem 9.Using the hydrostatic equation, and your pressure measurements from problem 8, determine the height of the 12th floor above the ground floor. For this problem, use a value for density of air of ρ = 1.05 kg/m3, and a value for g (acceleration of gravity) of 9.8 m/s2. Put all your calculations and answer on the answer sheet.

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Copyright © 1996-2007 Texas A&M University, Texas A&M Meteorology Department and Marion Alcorn.