How to Make A Barometer

Ever Wondered How To Make A Barometer?

It's Remarkably Easy And Quick, And Only Requires Material You Can Find Around Your Home

The Homemade Barometer Part 2, How To Make A Torricellian Barometer

If you have read about how Evangelista Torricelli invented the barometer in 1643, you've probably realized that his methods could be easily repeated, as indeed they were in school science labs the world over. However not only is mercury very expensive, but it is toxic and hard to dispose of safely. Also a 30 inch tube of mercury is not the most stable piece of apparatus to have around the home.

So we'll remove that idea from further consideration.

One small tip before going any further. Once you have the materials together, none of these homemade barometers described in the next few pages will take more than a few minutes to make. For best results try to assemble them when air pressure is in about the middle of its normal range - preferably not in the middle of wet and windy periods, which usually mean low pressure, or during episodes of still sunny days typical of high air pressure.

The first type of home made barometer repeats most of the principles behind the first mercury barometer, but is much cheaper and safer to make.

You'll need: Some water some food colouring to make the water more visible. A straight sided glass. A clear straw or similar piece of clear plastic tube A piece of white cardboard about 4" lond and 1" wide (say 10x2.5cm) Some clear adhesive tape or glue And some chewing gum.

Directions: Start chewing the gum while you stick the straw or tubing in an upright position on the inside of the glass. Make sure the bottom of the straw is not touching the bottom of the glass.

Add some coloured water to the glass until it is about half full.

Suck some of the water into the straw so that the water in the straw is well above the level of water in the glass but not too close to the top of the straw.

Now here comes the tricky part. Assuming you haven't swallowed the gum in the excitement of the moment, work the gum around so that you can press it over the top of the straw while both the straw and the gum are still in your mouth. The aim is to make an air tight seal at the top of the straw. If you do it first go you're a legend.

At this stage you should be able to see the coloured water in the straw at a higher level than the water in the glass.

Now draw a few evenly spaced lines across the cardboard, and stick the cardboard to the outside of the glass next to the straw, with the base of the cardboard level with the top of the water in the glass. You can continue the cardboard as high up the straw as you like.

Congratulations! You have made a water barometer. As air pressure changes it will act on the surface of the water in the glass. If it is higher than when you made your barometer, it will force the water higher in the straw, and if it is lower the water in the straw will fall.

By watching the level of the liquid over the next few days, and marking its position on the cardboard, you will have a record of air pressure changes in your area. By comparing your results with those from the nearest official weather station to where you live you can even develop your own scale on the cardboard.

There's one big difference between your barometer and the ones made by Torricelli and later scientists, apart from the lack of mercury. Torricelli had a vacuum at the top of his mercury filled tube, whereas you have only a partial vacuum. Using water, you would need a tube at least 35 feet or 10.7 meters long if you want a vacuum as well.

And while the theory is fine, your barometer won't stay accurate for long. Evaporation from the glass, temperature changes, and probably leaks in the chewing gum seal will all affect your results after a few days.

By the way, there are substitutes for the tongue applied chewing gum - modelling clay is just as good, but you will need to pinch off the top of the straw before you apply it.

Want another one?

Homemade Torricellian Barometers, Part 2

This is very simple and not at all elegant. All you need is a clear jar or bottle, the narrower the better, and a pan or a basin, and a couple of large carpenter's nails. You can add a ruler or a measuring stick, and some food colouring if you like.

Put an inch or two of water in the basin. Fill the bottle about half full with warm water - about as hot as you can stand, but not hot enough to burn you. You can use cold water instead, but your margin for error is better if it's warm. Invert the bottle into the basin so that the mouth of the bottle is below the water level in the basin. Rest the mouth of the bottle on the two nails so there is a gap between the bottle and the bottom of the basin.

Most water will run out of the bottle into the basin. As the water cools, so does the air in the bottle, and the water level will rise further - see the air pressure experiment above. Once the temperature has stabilized, the level of water in the bottle will rise with increasing air pressure, and fall as the air pressure falls. You can set up a scale to record changes in the level if you wish, but this crude barometer is rather unstable and takes up a bit of space.

Both barometers work because pressure changes on the water in the basin either force water into the straw or bottle when the pressure rises, or receive it back when the pressure falls.

If you want to know more about Home Experiments Related to Weather, just follow the link to an excellent book, and I know from experience it's not just for kids. And here are some other top introductory weather books.

Peterson First Guide to Clouds and Weather

The Weather Wizard's Cloud Book

More About Barometers

Barometers measure air pressure, so here are some links to pages on Air Pressure and Homemade Barometers. You'll find a few more air pressure experiments there too.

For more information on the sort of barometer you can make by following the instructions on this page, visit the article on the History of the First Barometers.

You can also make a homemade Weather Glass or Water Barometer, or even an Aneroid Barometer.

You can also find information on Setting and Calibrating Your Barometer, elsewhere in this site, together with reviews of Modern Barometers. This link will take you back to the Top, or, when you're ready, here's how to return to the Home page.

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