# How to Make a Barometer

A barometer measures atmospheric pressure. You can build a simple air-type or water-type barometer using common household items.

Let's start with an air-based -- or aneroid -- barometer. For this project you'll need:

• A coffee can
• Plastic wrap
• A rubber band
• Tape
• An index card
• A toothpick
• A drinking straw

Here's what you do:

1. Cover the can with plastic wrap. Secure the plastic wrap with the rubber band. Make sure the cover is airtight.
2. Place the straw in the center of the cover with about one-third extending past the edge of the can.
3. Tape the straw into place.
4. Tape the toothpick to the end of the straw that extends past the can's edge. Both the straw and the toothpick are horizontal.
5. Tape one edge of the index card to the can so the card extends out behind and parallel to the toothpick.
6. Draw a line on the card opposite the spot where the toothpick is pointing.
7. Add several evenly-spaced marks above and below the original line.
8. Check the toothpick's position every now and then. The higher it points the greater the barometric pressure, and vice versa.

[source: Aneroid Barometer]

Now let's make a water-based barometer. Here's what you'll need:

• A jar
• A yardstick or metric ruler
• A clear straw
• Tape
• Chewing gum or putty

Here's what you do:

1. Tape the ruler securely inside the jar with the numbers facing inside the jar.
2. Tape the straw to the ruler so that it doesn't block the numbers. Make sure the straw doesn't touch the bottom of the jar.
3. Fill the jar about halfway with water.
4. Draw the water partially up the straw and seal the top with gum or putty.
5. Mark the water level on the straw.
6. Check the water level in the straw periodically. A rise in the level indicates an increase in barometric pressure, and vice versa.
7. Calibrate the numbers on the ruler to the true barometric pressure by comparing it with local weather reports.

[source: Water-based Barometer]