What it shows
Air pressure changes from day to day. When the outside pressure rises it presses the balloon skin inwards and the straw pointer tips up; when pressure falls before rain, the pointer tips down.
The science
The stretched balloon traps a fixed amount of air inside the jar. On a high-pressure day the air outside pushes harder than the air inside, so the skin dips and the far end of the straw rises. On a low-pressure day the trapped air pushes the skin outwards and the pointer drops. Temperature changes the trapped air too, which is why the jar must stay in a shaded, steady place.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A glass jar or wide-mouth bottle
- A balloon with the neck cut off
- A rubber band
- A drinking straw
- A card with a scale drawn on it
- Tape and glue
Safety
For this project: An adult should cut the balloon. Keep the jar away from windows and heaters, because warmth also moves the pointer.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Stretch the cut balloon tightly over the mouth of the jar and hold it with the rubber band so no air can leak.
Glue one end of the straw to the centre of the balloon skin so that most of the straw sticks out past the rim.
Stand a card next to the jar and mark where the straw tip points on the first day, writing "start" beside it.
Draw small lines above and below the mark, each about 2 millimetres apart, and number them.
Read the pointer at the same time every morning and write the number in a table with the date.
In the same table write the weather you can see: sunny, cloudy, windy, rain.
After two weeks draw a line graph of the readings and colour the rainy days.
Look for the pattern: did the pointer drop a day or so before rain came?
What you should see
A graph of pointer readings over two weeks. In most places the pointer falls before cloudy or rainy weather and rises during clear days, which shows the barometer is working.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What does a barometer measure?
The pressure of the air around us.
2Why does the pointer tip up when pressure rises?
Outside air pushes the balloon skin down, which lifts the far end of the straw.
3Why keep the jar out of the sun?
Warm air inside expands and moves the pointer even when pressure has not changed.
4Which weather usually comes with falling pressure?
Cloud, wind and rain.
Take it further
Compare your readings with the pressure given in a weather app or newspaper for your city.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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