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Salem Clock Shop - 1085 Broadway Street NE, Salem, OR 97301 - (503) 581-3803 Fax: (503) 581-3331 |
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You can build a homemade glass jar barometer and use it to help make accurate local weather forecasts. |
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Glass Jar Barometer |
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Bill of Materials
· Glass jar · Rectangle of cardboard to fit · Length of wire or large twist-tie · Larger toy balloon · Rubber band · Coffee stir-stick · Toothpick · 3 X 5 card · Scotch tape & glue |
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Jar barometers are easy to make. Air trapped inside a jar expands or contracts depending on external atmospheric pressure. As the pressure changes, the surface of the balloon changes shape and raises or lowers the end of the stir-stick pointer. They can also be made from tin cans, but cans tend to rust. They are strongly affected by temperature, so keep it in a place where the temperature is near-constant or can be brought to the same temperature each time a reading is made. |
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Figure 1. |
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Obtain an empty jar (just about anything will do...the jar in Figure 1. is an empty 8 oz Maxwell House instant coffee jar).
Cut a rectangle of cardboard long enough to accommodate the jar and the 3 X 5 card, and wide enough to form a base under the jar and yet still extend well past the jar top.
Fold the cardboard as shown in Figure 1. Place the jar in position on the cardboard and punch two small holes through the cardboard behind the jar in a spot that is well-placed to guide the wire or twist-tie that will anchor the jar in place.
Scotch-tape the 3X5 card to the cardboard as shown in Figure 1. This is where the pointer measurement scale will be created later.
Cut the nozzle end off of the balloon and then cut along one side from the open end to the tip. You will then be able the flatten out the material.
Stretch the balloon fabric tightly over the mouth of the jar and rubber band it into place, making at least two loops with the rubber band. Trim off the excess fabric and wire the jar in place on the cardboard. |
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If you are building this during fair weather (i.e. high barometric pressure), you can place a loop of thread between the rubber band and the jar. Then, during the next low pressure period (rain or whatever), holding the edge of the balloon fabric, gently pull on the thread to draw the rubber band slightly and allow the current air pressure into the jar. This will also reset your pointer back to the benchmark. Then, proceed as above to create a measurement scale. |
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Figure 2. |
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Figure 3. |
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At this point, the membrane is as flat as it is ever going to be. Place a mark on the card at the current pointer position. This will be the benchmark.
If you are constructing this during stormy weather, all is well.
When the weather clears completely, the pointer should be in a considerably higher position. Make a mark opposite this new position. You can now make 3 or 4 marks between the two extremes as a rough scale. See opposite. |
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Snap the toothpick off near the end, and force it into the end of the stir-stick Color the tip to increase visibility.
Using a ruler, locate the center of the balloon membrane and place a drop of glue there.
Place one end of the stir-stick (the end without the toothpick) on the drop of glue with the other end pointing off the jar toward the 3X5 card as shown in Figure 2. |