Making a Barometer

INTRODUCTION

Air envelops the earth. The earth's atmosphere is approximately 370 miles thick.¹Air has mass, and the mass of the gaseous atmosphere exerts a force on the earth. This force is called air pressure. Air is most dense near the earth and less dense higher in the atmosphere. An aneroid barometer is a metal box that has been evacuated (the air has been removed from the box). The expansion or contraction of the walls of the box is then used to measure changes in air pressure. Airplanes have aneroid barometers on board to measure changes in air pressure with height in the atmosphere.
These barometers are called altimeters.²

MATERIALS

• 1 drinking straw
• 1 large-mouth jar
• 1 large balloon (11 inch diameter or larger
• Duct tape or very large rubber bands
• Tape
• Paper
• Manila file folder
• Toothpick
• Marker

WHAT TO DO

1. Blow up the balloon first to stretch, then let the air out. Cut off the neck portion of the balloon and stretch the remaining balloon material across the top of the jar.
2. Seal the balloon to the jar with duck tape or by using the large rubber bands. Make certain not to leave leaks between the balloon and jar.
3. Tape a toothpick to one end of the drinking straw. Tape the other end of the straw to the center of the balloon.
4. Fold the file folder so that it makes a large triangular prism.
5. Make a measuring scale (ruled piece of paper the size of a six inch ruler) and glue it on the file folder.
6. Use this ruler to monitor changes in the air pressure. The toothpick is the pointer at the end of the barometer that should point to positions along the ruler. If the pointer moves up, the air pressure has increased. If the pointer moves down the air pressure has decreased. Use this barometer to measure the barometric air pressure changes for an entire week.

QUESTIONS

1. When the air temperature decreases, the density of the air decreased. What direction do you expect the pointer to move for a temperature decrease?
2. If you took this barometer on a hike up a mountain, what changes in the measured air pressure would you expect to see?

EXTENSION

Check this web site http://weather.noaa.gov to obtain the barometric pressure at the beginning of the data collection. As the air pressure changes from one day to the next, monitor the changes observed with your barometer and check the reported barometric pressure for your locations. Make a graph of pointer locations versus the reported barometric pressure. A linear relationship should result if your barometer is working well.

SOURCE

"How Science Works." J. Hahn, Dorling Kindersley, London, 1991. ISBN 0-7621-0249-7
"Science for Children." 3rd ed., Williard J. Jacobsen, Abby B. Bergman, Prentice Hall, New Jersey, 1991.
"Teaching Chemistry with Toys." Sarquis, Sarquis and Williams, Terrific Science Press, 1995.

Grade Level: This experiment is expected to be appropriate for grades 3 and above.

© S. Olesik, WOW Project, Ohio State University, 2000.