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Barometric Changes: The Cause and Measurement of Air Pressure (page 2)

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Author: Janice VanCleave

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1.
1. Does atmospheric pressure outside the bottle barometer change enough to affect the water levels in the bottles? Make a scale by sticking a 3-inch (7.5-cm) strip of masking tape along the side of and centered over the mark of the top bottle. Use a fine-point pen and a ruler to mark a line in the center of the tape over the mark on the bottle. Continue to mark lines 1/16 inch (2 mm) apart above and below the center line. Number the center line 0. Number the tenth mark below the line –1 and the next tenth mark –2. Continue to the end of the tape. Repeat, using positive numbers above the center line. Set the barometer inside your home where it will not be disturbed. Leave the straw open. At the same time each day for seven or more days, record the water level on the scale in the top bottle.
2. Does the atmospheric pressure outside your house differ from the pressure inside? Repeat the original experiment, making a second bottle barometer. Then, repeat the previous experiment, placing one barometer inside and the other outside. Science Fair Hint: Record the atmospheric pressure from a local television or radio weather program. Prepare a diagram showing the bottle barometer's readings and the recorded atmospheric pressure for each day.
2. Does atmospheric pressure change during the day? Use the bottle barometer to record the water level and time every hour for 6 or more hours during one day. Use the information to prepare a bar graph showing the pattern of change.

1. Prepare another type of barometer by cutting the top from a 12- inch (30-cm) round balloon. Stretch the bottom section of the balloon across the top of a wide-mouthed I-quart (I-liter) jar, and secure it with a rubber band. Cut one end of a drinking straw to a point. Glue the uncut end of the straw to the center of the stretched balloon. Secure a metric ruler to another I-quart (1- liter) jar with a rubber band. The ruler must stand upright, with the zero end of the metric scale at the bottom. (The marks on the metric scale are close, allowing small pressure changes to be measured.) Position the two jars so that the pointed end of the straw points to the metric markings on the ruler. Do not allow the straw to touch the ruler (see Figure 28.2). Repeat the previous experiments replacing the bottle barometer with this jar barometer.
2.
1. Air is made up of gas molecules, mostly nitrogen and oxygen. These molecules move and hit against each other and anything that gets in their path. The impacts of these bouncing molecules cause pressure. Demonstrate how the impact of an air molecule causes atmospheric pressure by dropping a marble on a scale. Use a scale with a I-pound (454-g) capacity. Place the scale in the center of a box that is about 1 foot (30 cm) long and wide and about 6 inches (15 cm) taller than the scale. Hold a marble about 6 inches (15 cm) above the scale and drop it. (The box just needs to be large enough to contain the marble when it falls off the scale.) Observe the movement of the scale's dial.
2. Air molecules are in constant motion. Like air molecules hitting against a surface, enough marbles hitting the scale in succession would keep the scale indicator from moving back to zero. Design a method to drop a series of marbles, such as rolling them down a folded piece of cardboard toward the scale.

Get the Facts

1. Evangelista Torricelli (1608–1647), an Italian mathematician and physicist, discovered the principle of a barometer in 1643. An encyclopedia can be used to fmd out about the experiment that Torricelli did to discover how the pressures of air could be measured. A water barometer was invented by Otto von Guericke (1602–1686), the mayor of Magdeburg, Germany, in 1646. How did Guericke's barometer compare to the barometer designed by Torricelli?
2. Aneroid barometers are inexpensive and commonly found in homes and offices. Aneroid is from a Greek word meaning "without liquid." Find out how this barometer measures air pressure. For information about aneroid barometers, see Frank H. Forrester, 1001 Questions Answered about the Weather (New York: Dover Publications, 1981), p. 17.
3. Barometric pressure increases at altitudes above sea level. What causes this difference in pressure due to altitude? For information, see The Nature Company Guides: Weather (Time Life Books, 1996), p. 26.
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