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Activity Number 3 - Building A Weather Instrument
This project explains how to make a barometer to show changes in air pressure.
- Empty glass container or soup can
- Elastic band
- Adhesive tape
- Drinking straw
- Index card about 8 cm by 13 cm (3 inches by 5 inches)
- Cut a piece out of the balloon large enough to cover the top of the glass jar or soup can.
- Stretch that piece of the balloon tightly over the top of the jar or can and secure it in place with the elastic band.
- Cut the straw so that it is about 10 centimetres long and trim one end to a point.
- With the sharpened end pointing out, lay the straw on the balloon with the flat end at about the centre of the balloon.
- Glue the straw in place.
- Draw reference marks on one of the long edges of the card at roughly half-centimetre intervals. Tape the opposite (unmarked) side of the card to the jar, with the narrow end of the rectangular card extending above the jar top and the marked edge just behind the straw. The marked edge should stick out so that the sharpened end of the straw points to the reference marks.
Points of discussion
- The piece of the balloon that is stretched across the jar will act as a membrane. When the air pressure outside the jar rises, it will push down on the balloon, forcing it slightly into the jar. This, in turn, will cause the end of the straw to rise. Similarly, when the air pressure outside falls, the air pressure in the jar will be greater than the air pressure around it forcing the balloon to bulge slightly. This will cause the end of the straw to drop.
- You can chart the position of the straw against the reference marks on the card each day. This will not give you a numeric reading but it will tell you whether the air pressure is rising or falling. The pressure trend is an important tool in forecasting.
- Please remember to keep your barometer away from sources of heat such as radiators and sunny window ledges. If it is close to a source of heat, then your barometer will act more as a thermometer, with the air inside expanding and contracting to reflect changes in temperature, not pressure.
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